Is there a chart which gives estimates for how a tank should last? I had a 5# feeding my 55 gallon tank and I want to say it only lasted about a month....
Hate to say it, but you likely have a leak in your setup if you aren't running an incredibly high bubble rate.
There's no chart because it's so different from case to case that you really can't say. For example, how long a tank lasts depends on how much CO2 you are using daily. This depends on whether or not you run CO2 24/7, 8 hours a day, 10 hours a day, etc. Then, it also depends on how high your bubble count is. If you run 5bps, you will use up your tank faster than if you run 2.5bps. Then, it also depends on your working pressure. The amount of CO2 in one bubble at 30 or 40 psi could be more than the amount of CO2 in 3 bubbles at 15psi, so 1bps at 40psi is like 3bps at 15psi. Then, it also depends on how big your bubbles are. This depends on your bubble counter, the angle of the BC, and other things.
Do you see why there's no chart? =)
That being said, your 5lb tank should last you way more than 1 month unless you are running CO2 24/7 and using higher than 4bps at a reasonable working pressure. My setup had huge leaks awhile ago and it still lasted half of a month at 3-4bps and high working pressure, so I suspect there is a leak somewhere in your system.