How often do you need to replace the stones? Is there a certain number of knifes that it will do?
The coarse stones (100-200) tend to last for a few hundred sharpenings. The medium stones (400-600) tend to last up to 1000 or more. The fine stones (800-1000) tend to last up to 1500 or more. You'll notice an initial change with the coarse stones in that they will initially be very aggressive until the loose diamonds are knocked off the the base diamonds flattened a little. Once they reach that point, they're finally broken in.