The answer to this question depends on what kind of knife sharpener you own. A honing rod or honing steel should never need to be replaced, this type of device is moving the knife material to reshape it instead of removing material to create a sharp edge like a traditional manual sharpener. Knife sharpening stones or whetstones may need to be replaced more often as they hollow out and create cracks. When you see damage to your whetstone or sharpener, it's time to replaced it. A diamond or ceramic honing rod should be replaced every 5-15 years, a knife sharpening stone that lasts you a decade is an excellent deal.