From everything I've read, 20 mins a day should be the maximum, but if your light source is on the weak side, maybe more is OK ..... check how you feel, you'll know if you hvae to adjust time exposures ....
You are right about the 20 minutes but what does it actually mean.... So, the intencity of the radiation makes a difference as you mentioned (weak light source vs. powerfull one). Therefore, one should calculate how many joules per square cm one gets. 100mW/cm2 for 20 minutes is 120 Joules/cm2. With less power than 100mW/cm2 one needs more treatment time.
If you have a LED panel in front of you, the sides of the body don't get almost any radiation. I asked someone, who has worked with PBM studies, done one study himself and red lots of studies and he said that 20 minutes should be the maximum time per square centimeter with the power of 100mW/cm2. This means that every square cm of the body can be radiated 20 minutes. So, in my case, I turn 90 degrees in order to cover also the sides of my body, which should provide me approximately 80mW/cm2 to every cell of my body for 20 minutes a day from top of my head to my knees. Therefore, I am not in the upper limit yet with 4 x 10 minutes x 2 times a day.