In my case, I told myself that I have a whole 15# to run through before I have to worry. Sure, and then a month becomes 3 months, then 6.
There's really no good time estimate for how long CO2 lasts. If you clean your taps by forcing liquid through with CO2, purge a lot, etc, it goes at random times. I think the 20lb lasted me like 9 months and the 15 was about 8 months.