I understand from my reading on forums and various web guides that you should always use as much light as your ventilation (and budget) can afford. If your grow area maintains temperatures within -- god, is it like 60-80??? -- you can always use more light. I've also read on the forums that at a certain point, the light part of the photosynthesis equation can exceed the plants ability to acquire nutrients from the soil and CO2 from the air, and vigor will cease to improve.

I'm totally a newbie though and would welcome any critiquing of my forum knowledge. I'm just started my own little project right now.