nerdcore
Depends on the gun. It can vary by a lot.

Chonco the Sith Lord
I depends what type of gun, they usually shoot about 20 bb's before you need to reload.

Scamp
Not sure exactly but with my paintball pistol, firing occasionally, I can get about 32 shots per 12 gram at about 200FPS. Rapid firing I get about 22.

Jeff C
As long as you're free from any leakages in the magazine, you should be able to get off 25-35 shots before it runs out. Power of the shots will vary depending on the weather but amount of shots should till remain roughly the same (If its very hot out the CO2 will expand inside the canister causing a higher pressure and therefore more power when expelled from the gun, vise versa with cold weather)