What Nm is your Uv? UVA can be ran all day. It's what makes a CMH attractive IMO. UVB, depends on distance of the light. I would start at 30 mins a day for 5 to 7 days, and increase the time by 15 mins after that. Repeat until you see damage, usually the undergrowth. At this point, I would back off the UVB time by 30 mins as i know its limit.