Well at a guess a std atmosphere is about 14.5 psi so at 3000 psi you have about 200 atmospheres so presumably 200 times the volume of the tank. an airbrush will use about 5 CFM at 20psi so if your tank was 2.5 cubic feet it would use 2 atmospheres a minute and your tank has 200 so thats 100 minutes or about 1.5 hrs.
Some little compressors only put out about 1 CFM a minute so you can 4x that 1.5 hour figure i suppose in that case.
I could be wrong but that is a common sense answer from someone who dosnt know what they are talking about.....ie: me
I would point out that i don't know the laws in the US but in the UK i think your house insurance would need to know as well as the fire service if you had a fully charged tank indoors as in the event of a fire it could blow the house up. :woohoo: