I am wondering how to calculate when a compressed gas bottle of Helium will empty.
Pressure is a constant 500Kpa via a PRV
Temperature is constant ambient
If the PRV is keeping it at 500 kPa, then the answer is ... NEVER. Either the pressure is kept constant or the bottle is emptying. Your problem statement has a problem.
What do you mean by "air time" the gas is Helium?
What is the flow rate?
Is the flow rate constant?
Or what is the outlet orifice diameter?
What is the bottle volume?
What is the bottle initial pressure?
As I understand you are trying to calculate how long it will take until the pressure inside the bottle will decrease to 500KPan (4.9 atm). Well, it depends what is the outlet pressure (the pressure after the PRV) too.