I have a Mach 3 cart still on the handle with 75 shaves on it. I really need to get back to it but the Merkur Progress is difficult to say no to first thing in the morning. Granted somewhere in the high 20's number of shaves I began stropping it weekly on denim and that really seems to help.
If I shave 5X a week that's 15 weeks on one cart. If I could get that many shaves from each of 50 carts that's 750 weeks or somewhere just north of 14 years worth of shaving! (if I did my math right)
Now I'm attempting an endurance test on Gillette 7 o'clock Super Stainless. My previous high point was 15 shaves. The goal in this case is not to strop or otherwise help the blade, but only quit when it no long provides comfortable shaves. We'll see
After 10 shaves with a Mach 3 the razor is still going strong. My original goal was 10 but now it is 15. Only time will tell. I am not sure that I am getting any different quality of shave than with my fatboy and GSB. I do use brush and soap.