How much usable air in an air compressor?
You should just measure the usage time through real world experimentation. This will give you the quickest and most accurate information at your real world set of conditions.

Empirical measurement methods will factor in all the variables of the system that may not be considered in a simple theoretical mathematical model. In fact the empirical method would be used repeatedly to verify any mathematical model while all the unaccounted variables are factored into the model. 

If you do not currently have the air compressor unit on hand to do the empirical test you should consider one of:


See if you can borrow a similar compressor from a friend or colleague. It may be even easier to take your attachment gear with you to their shop or garage to do the test since for a test on a 3 gallon tank it will not take a long time at all. 
Rent a unit from a rental yard for a half day. Schedule it so you can use it when you can also plan to use it for a regular task. 
Take your gear with you to the orange colored BigBox store....many have rental sections....and explain that you are looking to purchase a unit from the store and ask them if they would help you run the empirical test right there in the store.