most important during flowering, but plants respirate throughout the veg process, and i have heard many claims of accelerated veg growth after introducing co2.
During veg and flower between 1100 and 1500 maintained. I've read a high times article were it stated that some believe it may reduce potency a bit if used excessively during the last 2 weeks of flowering, not sure about that. I just started my 2nd grow with co2, here is a pic of the trics off some BD that was co2 supplemented O check my avitar