If you are continuously bubbling CO2 (24-7) then 3 months is pretty average, IMO. The average for a 20# tank seems to be about a year, although this also depends on the original fill in your tank (amount of CO2). My last 20# tank lasted 16 months, but I use a controller and do not bubble continuously and had 100 psi to start rather than the usual 800-900. Dave