I am assuming they are dated and have a manufactures date stamp on it. Same as a propane tank. Once expired they probably wont fill it?
once expired you need to have them re certified. Which is a process in itself. Mine has an original date stamp of 1948. Just got it certified last year. New numbers stamped in it and off you go. It's about 8 inches by 5 feet.
I got mine from Praxair in Sydney over 2 years ago now. I'm not on a lease. I paid for the full bottle (between $50-$75, I don't remember) and use it until its dry. When it runs out I take it in and exchange it for another one for the same price.
53m38, stock for now except for a fiberglass tub 07 Taco TRD 12 Rav 4