I have been looking into setting up a CO2 system. The local fire extinguisher shop I checked with will sell me a refurbished hydro tested 10lb steel tank for $55. They charge $11 to refill it.

The short answer: it depends.

It depends on the size of the tank you are supplying with CO2, how bright the light is (which puts demands on amount of CO2 needed). And how efficiently you diffuse the CO2 into the tank. Less efficient diffusion typically means wasted CO2 gas escaping, and therefore you need to supply more.

I have been using a 5lb tank, making LOTS of mistakes as I learn and experiment. I've had a 5lb last as little as a week (leak in DIY bubble counter) on a 5.5g tank, and up to a month on a 20-long (slow leak in tube going to diffuser). Apparently, from others here, a 5lb on a ~20 gallon typically lasts roughly several months.

-Jason