The only way they can become ineffective is if they corrode. 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. Once again, the useful life of ammunition is theoretical. As long as your ammunition is safely stored and protected from moisture or sudden temperature changes, you should be able to keep it for decades. The useful life of munitions depends mainly on their state of conservation.
On average, ammunition can last at least ten years. However, your weapon may still be able to fire with the same ammunition even after a decade, as long as it is stored under ideal conditions. Ammunition companies are pushing a conservative message, probably because they don't want responsibility if it doesn't shoot (and, hey, they'd like to sell more ammunition). You may find a large amount of ammunition and stockpiles to use in the future, or you may not be able to make it to the shooting range immediately after purchasing ammunition.
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). But every time I find myself buying ammunition in bulk and adding more ammunition to the different types of ammunition out there.