I have used primers that are now 40 years old. They are pistol primers, and they all go bang. They have been stored in garages, shops, sheds and now in pistol brass. I have never had one not go bang.
I am not sure there is too much to the degradation of primers by moisture. If they dry out, they should still go bang. I have shot shotgun shells that spent three years under the floors of my duck boat. They all shot fine. They were not sealed primers like the Federal Premiums. I cut one of the shells open, and some of the shot was actually rusted together. They were No. 4s, so not much chance of barrel damage due to pellets going out lined up like would be the case if they were BBB or Ts. The ducks I shot did not fall, so maybe they were defective; however, my shoulder would argue otherwise.
Bart made the correct point I guess, "You may not notice anything until you get out 500+ or to 1k before you notice the difference."