I use a Mach 3, shave four or five times a week normally, and change my blade monthly. When I put a new blade in I do notice that I feel a little less irritation than with the “worn out” blade I just removed, but the overall shave isn’t really very much different.
My beard is not super heavy by any means, but I think it’s about average. So a month isn’t out of line at all. I could probably go six weeks without noticing it too much.