How long is ammunition good?

IF properly stored (cool, dry) I am not sure that modern smokeless ammo HAS a shelf life. I am shooting 8mm Mauser from the 1950s, and .303 Enfield from the 60s now. I have a few thousand rounds of 7.62 Russian from the 70s, and it shoots fine. Moisture and extreme heat seem to be the enemy. Late 1800s ammo used mercuric primers, and those seem to die as Mercury reacts with the brass casing, but later pimers do not seem to have the same problem.