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.