Most manufacturers guarantee that their ammunition will last at least a decade. That said, ammunition can easily last more than 10 years if stored under ideal conditions. There are countless stories of surplus military ammunition being used many decades after its manufacture. Modern ammunition is built to last more than a decade (and possibly up to twenty years), provided you follow proper storage practices.
Ammunition can last a minimum of 10 years and can last indefinitely, but there are several factors that affect its overall lifespan. These include the storage conditions of ammunition and current manufacturing practices. Ammunition manufacturers limit their liability exposure by being more conservative in their estimation of useful life (not to mention selling more boxes of ammunition by encouraging more frequent purchases).