This will help you.

http://www.hydroponics.net/learn/co2_calculator.asp
Zandor Reviewed by Zandor on . Co2 Tank Life I think I am going to be using a flow rate of about .3 cuft/hr for fifteen minutes on an hour in a 54 cuft grow space. What size tank should I purchase and importantly how long will it last. I would like it to last the whole grow hopefully. Rating: 5