Leave it in or take it out half empty?

Now this is the "how long is a piece of string question".....
So, myself, and this may be not the right thing, but will just say what I do....firstly is always use a quality cartridge ( branded Umarex or such like ) it is supposed - although I can neither prove or disprove, the better the cartridge the better the fit and allegedly the better cartridges are better for the seals - but again - cannot prove that sat here right now.

Lubes can be used for the seal / nipple - but I have never used one - some say it's a good thing - just don't know.

I would happily leave the CO2 attached in a pistol type canister setup for 4 - 5 days at room temperature - not in the garage or shed. I have no scientific reason for that, just my brain saying don't leave the CO2 in it too long.

In a rifle - well its a bit more sturdy, seals and retaining may be better, but again I would leave CO2 in mine for max 4-5 days - again no evidence its just what I do.

Thats it, probably pointless - but I offer what I do for what its worth.

Happily be proved totally wrong - I do normally shoot, finish the cartridge and then put away.

opinions will vary but the general rule is not to leave a CO2 cartridge in the gun for long extended periods of time. some will say to immediately remove the cartridge regardless what may still be left at the very instant you're done shooting even if you planning on shooting the gun again the next day. Others, like me, will leave the cartridges in the gun for several days, maybe even a week or so, with little concern or problems. And then there are others that have left cartridges in their guns for a year or more and claim no ill affects.

Of course it depends on the gun. For guns like the Crosman 22xx guns I wouldn't recommend leaving the cartridges in for very long extended periods of time. These guns use mechanical pressure on the cartridge to create a seal against the valve. The seal may take a memory set to that cartridge if left for a long period of time and the seal may not work with the next cartridge when you finally do replace it. But like I said, I may leave them in my 22xx guns for a week or so and I've not had any issues. If your gun is like the first variants of Crosman 600 or Mark I where the tube is pressurized after you puncture the cartridge I would say you could probably leave them in longer. The cartridge isn't pressing into the seal or valve in those guns and they are more like bulk fill CO2 guns once the cartridge is pierced. there is an argument that the CO2 gas itself may cause seals to deteriorate with long periods of exposure however. I think it depends what hey are made of.

There is of course the safety aspect to consider as well. Probably the first thing and most important thing to consider. Just keep in mind if you do leave pressurized cartridges in your gun it is in effect "loaded" and they can shoot out anything that maybe in the barrel or put down the barrel. So keep them stored safe and secured and away from those that ought not be handling them unsupervised. If there are laws prohibiting it or a concern about and unauthorized user having access to it that could be messing with the gun unsupervised I would suggested never to store the gun with a pressurized cartridge for any period of time. You are after all the owner of the gun, therefore ultimately responsible for it and what happens with it.

One important thing to do, is to put a drop of Pellgun oil on the tip of each cartridge before installing it. the guns seals will thank you for it. Some will say can do this every 2,3,4 or whatever number of cartridges (again, opinions vary). I do it every single time. That way I don't lose track when I last used the oil. just simpler that way. maybe this is why I haven't had any issues with my seals either.