After a few years, I would replace a sharpening stone. If you run your finger across and feel any grooves or just can’t feel any coarseness, it’s time to replace. I mention the grooves because I just feel grooves can harbor bacteria (I’ve not seen any studies on this though). Also, if you feel your instruments aren’t getting sharp by checking on a plastic stick, but then after using a different stone the instrument is sharpened, its time to replace. It is dependent on how much you sharpen, but the point is, they don’t last forever! I hope this helps!