You can re-sharpen a set of Lister blades up to 10 times, dependent on blade condition, maintenance and usage. For example, if the blades are tensioned too tightly and are not oiled, the cutting action will wear the touching surface away, reducing blade life. Professionals will tend to replace their blades more regularly due to desired finish. As the blades are sharpened, the lead on the tooth grows meaning the closeness of the clip will change over time, so an A2F/AC (Fine) blade may give the same results as a A2/AC (Medium) blade after it�s been sharpened a number of times. Therefore, as professionals will still require the close clip, i.e. 1.4mm of hair left, they will need to purchase a set of new blades sooner as the re-sharped blades may not give them the same finish as a new set would.