Gary,
I can't discern your spraying habits, but I can give you the basic math to help you figure out approximate spray time. Most full size gas bottles (the ones you see in oxyacetylene torching setups, for example) have a maximum volume of about 6 cubic feet. Making an assumption that your bottle is about half that size, then the volume should be around 3 to 3.5 cubic feet. If you multiply 3.5 cu.ft. times the pressure on your regulator (1,000 psi) and then divide by 14.7psi at sea level to adjust for standard pressure, you get 238 standard cubic feet. If your airbrush is being powered at, let's say, 10 cfm, then your total (continuous) spray time is only going to be about 24 minutes before you're going to need a refill.
I hope this helps you answer your question.
Depending on how long and how often you spray, the cost of refills, and the trouble you have to go through (traffic, distance, gas, etc.) to get your refills, you may want to consider a small compressor, or a compressor with an attached tank, if noise is an issue.
Also, keep in mind that your cylinder is required to be hydrostatically tested every 5 years. Unless you have a maintenance agreement with your CO2 supplier, that would, I assume, be an additional expense.
Gip Winecoff