Hi Cindy & Linda
        Depending on what model airbrush you have, you should be able to calculate
the air time your tank will provide.  Look up the cubic feet per minute used
at what pressure you'll be working at.  I think the aztec airbrush uses
.25cfm @ 25psi, but I have a biodegradable memory....
        If you are doing fine  work, a tank will last a good long time.  If you are
painting backgrounds or murals with a higher volume airbrush, the tank will
empty much quicker.  Although I use a silent compressor, I bought a second
stage regulator at Crow-Burlingame, and I use teflon tape on all the
connectors to stop those pesky leaks.
        There was a fellow who painted a mural in a park downtown; his airbrush was
powered by a spare tire!
        Happy painting |:-)
        Darcy