Longer than you need to worry about, while in the Marines a few years ago it was routine for us to shoot off Korean War vintage .50 ammo without a hitch. There are also numerous stories of Grandad's WWII Colt reliably firing off all that was in the magazine....
+1 Dirty D...
You can still find and shoot WW II ammo....
I rotate my carry ammo every couple years if it hasnt been shot and use the old stuff at the range. (mainly because I like the look of shiny new Speer Gold Dots)
target ammo I keep forever or until I am ready to shoot it.....
i am not woried about "stale" ammo
I sshot some 1938 8x56 yesterday. had no misfires or hang fires or no fires. good ammo and great gun. dont have a clue how long ammo will last but 1938 ammo is still good, so if you bought it while you are alive i think it is ok. that is so long as you store it in a cool dry place.
I heard about some guy handing out some 1920s-era .303 British to the SMLE shooters at his local range that had been stored in a wooden crate outdoors without moisture protection and only a handful didn't fire.
Read somewhere else about some guys in an Army warehouse finding some interwar (1930s?)-era loaded 1911 magazines in the 1980s and shooting them off without a hitch (both the magazines and ammunition performed flawlessly) - again, no special storage arrangements.
If you store your ammunition indoors with climate control, odds are it will outlast you.
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