I have a few boxes of Remington .38 special that my grandfather gave me with his old pistol. The ammo is from 1974. The department store he bought it from here in Florida went out of business in 1978 (I believe).

They still work fine. The first time I shot any, I hid behind a concrete block wall with just my hand & the pistol hanging over the top. I popped off 6 rounds, counted my fingers, didn't see any blood & called it good.
I also buy military surplus ammo for my SKS & AK. The ammo is from the late 70's / early 80's & still works fine.

I can't see any reason why it wouldn't keep for a good long time as long as it's not sitting in water.....