The Coleman-style cylinders stay fresh forever (it seems) as long as humidity doesn't rust the connection surfaces.

You could probably sell them on Kijiji/Craigslist and get at least half your money back. It's pretty obvious if they're full or not.

They also have the same thread as the standard hardware store propane torches (bernzomatic or other brands). Handy for soldering, fire lighting, unseizing rusted bolts, all sorts of stuff. Or making creme brulee if you have culinary inclinations.