If I puncture a CO2 cartridge, will it last a while? I bought a CO2 cartridge a while back.
I think it is used to fill up tires.

If I puncture the cartridge, will it last a while?
Would it maintain pressure for at least 6 months?
Think of these as a one shot device. Once the seal is cracked, any remaining CO₂ will leak out over time, and you have no idea just how much is left. .  

How much time I have no idea, but the problem with it is as you have no idea how much is left in the cylinder (which is as likely as not no where near enough to pump up the tire), you have to carry enough cylinders in case its empty.  If your carrying enough in case its empty, you are carrying extra weight. 

While I do carry one of these, on long trips (more than 30minutes ride from a road with cell phone coverage), I carry a real pump.  Between the risk of multiple punctures and loosing the gas from incorrect application (Don't laugh it happens surprisingly often)  I see these are a time saver, not a life saver.