Okay - I run a 4x4x7 tent, and a 20lb. bottle of CO2 lasts me about a week, keeping it steady at 1500PPMs (Sentinel CHHC-1, and the CO2 only runs when the lights are on during 12/12).
I got tired of filling up the bottles (expense, and a security issue), so I bought the CAP Gen-1 generator...