Ammo stored correctly will probably last 100 plus years. I am still shooting my 1942 dated case of 8mm Mauser ammo from Turkey. Came in a big sealed tin though. Stuff looks as good as the day they made it, no duds or hangfires, and accurate. I also have a small stash of FN made .30-06 M2 stuff made in 1957 and it shoots 1" or better at 100 yds. (best shooting surplus ammo I have ever had though). A few years ago, I got a 5 gallon bucket of old ammo from a co-worker who cleaned out his uncles house after he died and literally had ammo in every drawer and place in the house. Hundreds of rounds with about 30 different calibers. Had a bunch of old 38 S&W rounds, and I figured I would burn them up in my old break top Enfield. Most were head stamped WRA, back when Winchester made ammo, before it combined with Western ammo. Funny thing, bang, bang,bang, Boooom-lots of noise, big puff of white smoke. about half of the rounds were black powder rounds yet. I have no idea of their age, but at the newest they were pre WWII, as that is when most if not all ammo makers quit making black powder loadings. They all went Bang!!
Watch out though when shooting really old rifle ammo, as brass does get brittle with old age, and split cases can occur upon firing.