About two weeks shaving Monday to Friday with a Mach 3. I could go longer, and often did because of the cost, but it would feel noticeably duller after that. I'll be honest, I never had an issue with the quality of shave I got with a Mach 3, it was the cost of blades that initially got me into DE. Of course I spend tons more on shaving now, but its enjoyable rather than a chore. But I reckon that if I went back to cartridges with what I've learned about prep, I'd get a very good shave.