Airsoft Kwa M9 Mag

How long do I charge an airsoft GBB pistol mag for? (Propane)?
I have a KWA M9 PTP Gas blowback airsoft pistol, and I can’t figure out how long to charge it with propane. I’m afraid of doing it for too long, yet only doing it for a few seconds gives me about 4 to 5 shots.
Help is appreciated!
Thanks Dmitri. Another problem: the rubber o-ring is coming off the intake valve on the bottom of the mag. If it’s not on about half of the propane escapes when filling. How can I make sure it stays in?
You can safely charge the magazine until you hear gas escaping. Keep your seals in good shape and you won’t have a problem.