Propane has an insanely long shelf life. I usually top my tanks up prior to storing for the winter, that way I have one less thing to worry about come spring.
We were cleaning out one of the old barns on the property prior to disassembling it. Found a couple of old (1950s) LP tanks that used to feed the cookstove in the old house. That house has been gone since 1968. The LP in the tanks was still good, used it to run my LP space heater.
Aaron