I don't know any mean average, but NATO, aka lifeboats will eventually degrade. Sealed canisters will of course last longer. I replace mine every 2 years and the old stock is still fine. Anything that is disposable in use, matches, water purification tablets, meds etc. should be treated as a finite resource with a finite shelf life. Erron the side of caution. If anything, you get to practise <img src="/images/graemlins/wink.gif" alt="" />