How long would a 12gr canister of CO2 last an airsoft gun?

I had a S&W SW40F blowback and I could get about 3 clips of 40 rounds. The KSC Uzi blowback uses 1 for a 40 round clip but can be improved to do about 2.5. It depends on 3 things, the weather (temperature air pressure and possibly humidity) the efficiency of the gun, and the construction. If you have a blowback that will use more air, if you have a solid metal gun that is blowback that will use more. Look at reviews on youtube and the web for the gun you are looking at and they will usually say how efficient it is.