Thanks for the replies. I also received the following information from tech support at Genuine Innovations in the States
We’ve performed several tests regarding leakage and, on average, the co2 will remain in the cartridge after puncturing for about 6 months. Of course, you must leave the cartridge attached to the Proflate Elite. In fact, as long as there is pressure in the cartridge, the safety feature of the Elite won’t allow you to remove the cup. It is pretty much the same with all our co2 inflators, too.
We don’t guarantee the co2 will last that long due to dust, dirt and physics (the molecules are pretty small so they will escape over time.)
Personally, I carry a spare cartridge, but I’ve never been let down. There was always some left over so I could top off.