Big Cholla

Rifle and pistol loaded cartridges loaded by a factory will be good for many, many years if stored properly. Obviously avoid exposure to water and excess heat. Ammo likes the same conditions that you do, dry and cool-not hot. Shotgun ammo will last for years also, but can develop a problem if it is the older version loaded with nitro based powder. The nitro can settle to one side of the powder charge over a long period and cause unsafe firing condition. Modern shotgun ammo probably is quite safe for 5 years and maybe even 10 years if kept dry and cool. Military surplus ammo is usually good for years if loaded in the USA, Germany, Britain and Russia. Really old surplus ammo should be avoided.

Edit: I forgot to mention to avoid exposure to ammonia fumes. Brass will react with ammonia and becomes very brittle. In other words, the brass looses its elasticity. A rupture of the brass case within the chamber becomes much more likely if the brass cartridge has come into contact with ammonia in any form. Many years ago there was a case of a reputable manufacturer in Brazil purchasing the cardboard for their cartridge boxes from a new manufacturer. The new people were using a paper curing technique that used ammonia in the process. It was found after many of those cartridges burst within the firing chamber that the cardboard boxes had just enough residual of ammonia retained to create a problem for the brass.