5000 lumens/ft2 is the figure I see all the time as the minimum for optimal flowering. The data on lighting is bewildering and everyone seems to have their own ideas. I'm just regurgitating the common info that I've come across. Also I don't use cfl and things may be different when using that kind of light. The other variable here is distance from the light source, closer=more lumens. When people rate a bulb by lumens I believe they are measuring at 1 ft. distance. Hopefully someone with more experience with this (esp. cfl) will answer. But you should google it. There is a ton of info about this out there. Also the Grow FAQ on this site discusses this.