Stored properly, modern ammo should last more than 50 years.  But proper storage is the key.

What is "proper storage"?  It is an airtight container, such as a USGI ammo can (who'da thunk it?) stored in a cool, dry location.

Ammo stored for a year in a car trunk or steel shipping/storage container, where summer temperatures can cause the ammo to bake at 130-140+ F, can cause ignition failures and lower velocities because of deterioration to both the primer and the powder.

Ammo exposed to moisture will corrode, and ammo that isn't sealed that is submerged in water will get water inside the case and ruin the powder and primer.

Ideally, you want to store your ammo in ammo cans (make sure the seal is good) at 60F or lower.  Up to 80F isn't likely to hurt anything.  Over 80 degrees and you will eventually (over a period of years) see some degredation, usually lower velocites and higher standard deviations.

-Troy

You can usually see if the can is bent, the gasket is torn, or if the contact line between the can and the gasket is far enough from the center-line of the gasket that the can isn't likely to seal.  It is also a good idea to add a thin film of vaseline on the gasket to make sure it doesn't dry out (dry-rot).

-Troy