How long does an air rig keep pressure from a co2 canister?

If I load a co2 canister (12g) into an air rig, how much "life time" can I expect from it, normally? I heard a minimum of pressure loss is normal between QD and regulators, because that is how they keep the pressure to consistent levels..

Anyway, if I load a co2 canister and leave the rig be as is, connected to my magazine..and go sleep..how long should it stay at the desired PSI?

Thanks!