This is a great question, but the answer is not so simple for a variety of reasons such as:

How powerful is the device?
What distance are you from the LEDs?
What are your treatment goals?
What wavelengths do you want to use?
What is your skin type/colour?
What is your genetic sensitivity?

Short Answer:

As will be shown below an exposure time of between 5 and 10 minutes
on a modern LED panel will produce 20 to 40 joules of treatment, which
appears more than adequate for most recommended treatment protocols. The
closer you position yourself the higher the power you receive, and when
further away you may need to increase the session times.  You should
also adjust based on your own response to treatments.

Long Answer:

With all phototherapy safety should be the #1 consideration so here
at Dermfix we think a good starting point is first to guard against the
possibility of overdosing on red light therapy.  Fortunately, there is
some good clinical data (link at the bottom of this article) discussing
Maximum Tolerated Dose (MTD).  The short text conclusion is as follows:

“LED-RL is safe up to 320 J/cm2 for skin of colour and 480 J/cm2 for non-Hispanic Caucasian individuals.”

Joules are a unit of energy, and this can be calculated if we know
the output irradiance, time used and distance from the source.

A typical modern LED panel will have an irradiance output something like the following at the centre:

80mW/cm² at 15cm
65mW/cm² at 30cm

Note: Due to the way light travels and crosses, you will always have a
stronger output in the centre of a panel and this naturally decreases
towards the edges.

At 15cm every minute you use the device would therefore add 4.8 Joules of energy (per square cm2) to your body.
Maths: (60 seconds * 80 mW/cm2 = 4.800 mW/cm2 = 4.8 Joules)

At 30cm every minute you use the device would therefore add 3.9 Joules of energy (per square cm2) to your body.
Maths: (60 seconds * 65 mW/cm2 = 3.900 mW/cm2 = 3.9 Joules)

So, we can also now do some quick calculations for the 15cm closest distance and using the highest output figure at the centre:

320 J/cm2 would be 320 divided by 4.8 = 66 minutes maximum exposure for skin of colour

480 J/cm2 would be 480 divided by 4.8 = 126 minutes maximum of exposure for non-Hispanic Caucasian

These times would increase when further away at say 30cm.

Note: These exposure times above are much greater than recommended
for any treatments but we hope should be reassuring for most users in
regards to a maximum tolerated dose.

So how many Joules per square centimeter (cm2) should I aim to get?

As a general guide for most treatments something between 5 and 80
Joules per square centimetre (cm2) of your skin seems more than adequate
based on clinical trials.

The maths therefore would be:

1 Joule = 1.000 mW/cm2.  Divided by 80mW/cm² at 15cm would be 12.5 seconds
5 Joules = 5.000 mW/cm2.  Divided by 80mW/cm² at 15cm would be 1m 2 seconds
20 Joules = 20.000 mW/cm2. Divided by 80mW/cm² at 15cm would be 5m 22 seconds
40 Joules = 40.000 mW/cm2. Divided by 80mW/cm² at 15cm would be 8m 20 seconds
80 Joules = 80.000 mW/cm2 . Divided by 80mW/cm² at 15cm would be 16m 40 seconds

Or at a further distance:

1 Joule = 1.000 mW/cm2.  Divided by 65mW/cm² at 30cm would be 15 seconds
5 Joules = 5.000 mW/cm2.  Divided by 65mW/cm² at 30cm would be 1m 16 seconds
20 Joules = 20.000 mW/cm2. Divided by 65mW/cm² at 30cm would be 5m 7 seconds
40 Joules = 40.000 mW/cm2. Divided by 65mW/cm² at 30cm would be 10m 15 seconds
80 Joules = 80.000 mW/cm2. Divided by 65mW/cm² at 30cm would be 20m 30

Safety of light emitting diode-red light on human skin: two randomized controlled trials

This is also a good question but with no 100% clear answer.  Based on
most clinical trials we have seen the 20 to 40 joules of treatment
should be completed a few times a week. Something like 3 to 5 times a
week seems optimum as it also leaves the skin a few days of rest.
 Whilst we have not seen any negative data on daily usage, it seems
logical to allow your skin some rest days.

This appears from the research to be not critical. We have heard from
users that prefer early morning red light sessions to improve energy
levels, and others that prefer evening usage.  Some also use red light
before their training sessions to enhance performance, or indeed after
training for recovery.  We recommend each user experiments and finds a
protocol that suits their own daily routine.

Yes, eye protection is recommended as the LEDS are very bright -
although the wavelengths used are themselves not considered dangerous.
 Some research has shown red, and near-infrared may be beneficial to the
eyes but here at Dermfix we do not feel enough treatment protocols have
been demonstrated to be able to recommend this as a treatment.  If you
do choose to use a red-light panel without using eye protection, then we
recommend you do not stare directly into the LEDs.

The standard Dermfix RLF LED panels are not certified beyond an IP20 rating
which means they are not suitable for bathrooms, outdoors, saunas or
anywhere there is a risk of water or steam contact.  Suitable areas
inside the house are bedrooms, living rooms or similar. But we do also have our RLF SPA panels that are IP65 rated against water and humidity and these are suitable for damp and hot environments.

Absolutely.  Here at Dermfix we are aware of the benefits of some
sensible sunlight exposure (Vitamin D production etc) but red light does
not have UV-A and UV-B and therefore can be used in a safer and more
regulated manner than exposure to natural sunlight.  Like many things in
life, a little of both can be good.  Red light can offer some added
benefits such as increased energy levels, muscle recovery and even added
protection against sunburn so is suitable for use throughout the year.

Red light therapy is thought to work by acting on and assisting the
“mitochondria” in the cells of your body. The mitochondria produce ATP
(adenosine triphosphate) energy for the cells to utilise. With more
energy your cells can do their work more efficiently, doing things like
repairing skin, boosting new cell growth and enhancing skin
rejuvenation.

Research is always ongoing, but red-light therapy is being applied as
a treatment for some common skin conditions, including, but not limited
to:

    To assist wound healing.
    To help clear acne, psoriasis, rosacea, eczema.
    To lessen scars.
    To improve sun-damaged skin.
    For hair growth.
    Reduce stretch marks.
    Reduce wrinkles, fine lines and age spots.

This list is constantly expanding.  Please refer to our NEWS section
for updates on specific conditions being treated with red light but some
examples are:

To reduce cell inflammation.
To increase blood circulation to the tissue.
To stimulate collagen production
To Increase fibroblast production 
To Improve skin health
To Increase energy
To aid Muscle recovery
To Improved circulation
To Reduced inflammation & pain relief
To Boost melatonin
To offer some pain relief

The RLF units are fitted with dual LEDs so that the 7 wavelengths are
spread as evenly as possible across the LED panel.  Each LED is made of a
combination of 2 wavelengths and this leads to colour differences when
viewed with the human eye.

Red light for home usage with LEDs is still new science and
advantages are constantly being discovered for different wavelengths.
 The modern panels therefore allow for adjustment of each individual
wavelength allowing for individually tailored modes.

In the past most of the research was conducted at around the 630nm and 850nm wavelengths, but this has now expanded. 

The individual wavelengths can be summarised as follows:

Blue

480nm blue light is commonly used in
light therapy for skin health, particularly for treating acne and
related skin conditions. It is believed to work by targeting and killing
acne-causing bacteria on the skin's surface. Additionally, blue light
at this wavelength may have anti-inflammatory properties and can help
reduce redness and improve skin tone

Red

These wavelengths range from 620 to 750
nanometers (nm), are visible and absorbed into the skin to a maximum
depth of between 4 and 5 mm.

630 nm: Perfect for stimulating collagen production and skin
rejuvenation. It sparks fibroblast activity in the upper epidermis and
dermis.
660 nm: Goes a bit deeper to reduce inflammation and support wound healing in skin and shallow muscle tissue.

Near Infrared (NIR)

Near-infrared (NIR)
light, which is above 700nm penetrates deeper into muscle, joints, and
even the brain, where it supports pain relief, inflammation reduction,
and cellular energy production.

810 nm: Known for boosting
mitochondrial activity and energy (ATP) production, which is ideal for
muscle recovery and metabolic support. studies have shown that combining
the 810nm wavelength with other wavelengths like and 1064nm can lead to
improved skin rejuvenation outcomes, including wrinkle reduction and
increased collagen production.

830 nm: 30nm near-infrared light is used in light therapy for skin
health, particularly for its deep tissue penetration and ability to
stimulate cellular activity. This wavelength is known to promote skin
healing, reduce inflammation, and boost collagen and elastin production,
leading to firmer, more youthful-looking skin

850 nm: Delivers deep penetration, targeting joints, ligaments, and
deep tissues for pain relief and structural repair. Near Infrared
(850nm) is designed for deep tissue, muscle, and joint treatments.

1060nm:
often used in non-invasive skin treatments, and can be beneficial for
skin health in several ways, including facial rejuvenation, fat
reduction, and treatment of vascular lesions. It's a wavelength that
penetrates the skin to target specific issues like fat cells or blood
vessels.

Ideally not.  Red light is blocked by clothing and most of the
 near-infrared is also blocked.  We recommend wearing as little clothing
as possible  or ideally none at all!  Clean dry skin is best and with
no lotions or creams applied (they can be applied after treatment).

Pulsing is the ability to rapidly switch the LEDS on and off…..something
like a strobe light effect. The latest red light LED panels offer this
option to pulse between 0-9999hz.  Here at Dermfix we consider this
option is only for advanced users, and we have not seen enough clinical
data to justify recommending its usage or enough guidance on possible
treatment protocols.  So we have integrated this option into our panels
so that you are future proofed, but for most users we would not
recommend using this option.  We will of course update this page in
future as more information becomes available.

These simple and convenient buttons allow for a range of quick everyday
settings.  Exact details of the settings are included in the
instruction’s manuals included with the LED panels, showing the
different percentage of wavelengths and the times that are used for each
smart mode.  We think these are the quickest and easiest way to use the
devices once you have a chosen mode that suits your lifestyle and
needs.