The stones that ship with your sharpener should last you through many sharpenings. The biggest factors are the hardness of the metal you are sharpening and number of knives you routinely sharpen. Our testing has shown a very long useful life for all of the stones, often exceeding 500 knives.
How will you know it's time to replace them?
You’ll be able to see bare spots on the stone where the diamonds have rubbed off. If you’ve already cleaned the stones well and they’re just not performing, then you might be getting close to needing to replace them. White vinegar with a little nylon scrub brush works well to clean them up and remove the metal buildup between the individual stones.