When I put in a new blade and then run over a pin and nick it. Oh yes, I have done this. I save the old blades and put them into an old cutter and use on paper.
Seriously, when I can't cut through 4 layers comfortably, that is when I replace. I've noticed since I now use the Martelli Ergo, I do not replace blades nearly as often, I think it has to do with the angle I use when cutting. With the straight cutters, I always seem to catch the blade on the ruler edge. I know I am weird. grin