Hi Lew...Rwise is spot on. Generally, during the vegetative state you want your light on at least 18, 20, or even 24 hours a day. I like 18 on 6 off for veg. that schedule gives the plants some rest time. It might be a personal preference. For the flowering stage you want 12 hours on 12 hours off. The distance the light should be from the plant is determined by the manufacturer.