only frickin works if you fill the EMPTY at some fricken point while you're busy sucking down beers! Ugh... I have an empty 20 and 15lb now and beer not flowing so much. Epic failure.
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.
I feel your pain. I had a #20 bottle (full) that was emptied in about a week due to a leaky hose. Now that it's been fixed I have 3 -#20's and a #10 on standby.