I can get through round about 20 blades with a full days workload. One of the obvious ways to tell a blade is becoming blunt is that it won't cut as fast. Another way to tell is that it is more difficult to keep the blade on the line of the pattern. If you use flying dutchman blades they never wear out, they break long before that. A selection of good quality blades, like Olsen or Pegus may seem expensive, but they do last and I can't recall having one break on me. As Barry said, it's so easy to put a blade in upside down and you will soon see the difference when you start cutting. I will give you a tip here. I keep my blades in test tubes that have a rubber bung in them. When I get a new order of blades I put them in the tubes so the tops end of the blades are at the top end of the tube so when I replace a blade i know straight away which end is which.