Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
There's a LOT of variables. Ignoring them all to provide a VERY rough estimate, maybe 3-4 months for a 20lb.
Before considering a 50 pounder, make sure it won't be too much for you to lift. Should you have a relatively small leak, you can lose even 50lbs. of CO2 in just a few weeks, it's less painful to lose 20lbs. Should you have a massive leak, I think I wouldn't want to be in your house!
Two 20lb. tanks could be an alternative. When one empties, you can have it refilled at your leisure, instead of rushing to go do it.