I believe that the shelf life is upward of 5 years, but that might only be for the higher end packaging with sealed foil. Any air exposure at all will allow the reaction to proceed, so you want to make sure they aren't treated too roughly for long term storage.
There are several manufacturers, so that might not be the case for all of them. I've seen some in cheap plastic, and some in heavy foil, so I know which one I would buy for long term. <img src="/images/graemlins/smile.gif" alt="" />
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Experience is a hard teacher because she gives the test first, the lesson afterwards.