I still have a bunch of circa 1937 7x57 ammo made by Kynoch and put into a 1000 round spam can. It was in the can until 1992 and has been in a good, stable, dry environment since then. Still shoots great and is very consistent over the chronograph. If kept in a good environment, it seems most ammo will last many decades. Worst stuff I ever had was some old 303 British mil-surp that looked pretty grungy so I figured it wasn't stored too good. It all went "bang" but wasn't as accurate as I expected; upon chronographing it was found to have very large velocity variations from shot to shot.
