Not a problem. Blue wave is in the visible range. It is ultra violet that can have adverse effects. As in all things, listen to your body. If you, personally, have an adverse reaction shorten exposure. See wikepedia entry on light therapy.
Depends... Blue light therapy can be used in the treatment of skin disorders (vitiligo, psoriasis, atopic dermatitis) and for mood disorders including depression. As to how much can you take per day is best answered by both the provider prescribing it for you; and the product manufacturer.