I choose to replace them if : they are badly beaten, bent, sails starting to wear away from the outside edge or have been sharpened to the point that a proper sharpening would shorten the blade to the point of creating quality of cut issues. Or, you could apply convenience or monetary reasons. If you pay someone $20 to sharpen and a new set is $35 and you don't have to wait for example. An extra set of blades is a good way to go anyways.