I change when I make a cut and have skipped spots. I don't re cut several times, that's a waste of my time. A sharp blade will cut the first pass. I keep several roatry cutters with new blades in them available to grab. The cutters that need a new blade go in a container until I have time to re blade them. When I find a rotary cutter on clearance I buy it. I have found that the Gingher rotary cutter with Gingher blade last almost forever. I haven't changed that blade since I've bought it.