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. 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.
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. Most manufacturers will tell you that ammunition has a shelf life of at least 10 years. The truth is that with proper storage, it will last much longer than that.
The key to storing ammunition is to keep it away from high humidity in a temperature-controlled location. Moisture can cause corrosion of brass housings and copper projectiles so much that the cartridges cannot accumulate or be removed after being fired. If you live in an area where the average humidity is above 50 percent, you should solve this problem by using a dehumidifier or finding another way to absorb moisture. 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).
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). 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. There is practice ammunition that is used to drill holes in paper, sound steel, or break clays, and there are serious ammunition used to hunt or defend oneself.