It has already been mentioned but it is hard to create a solid rule. I use the Astra SP as my primary blade. I am bit of a tight wad so I use one blade per week. - I have a mild beard. Four passes on Sunday, Wednesday and Friday - Two pass on Monday, Tuesday, Thursday -no shave on Saturday.
I used to get 6 weeks out of the Merkur blades. Of course, I used to get a lot if nicks too. I gave up Merkur blades for better performing brands. Now 7 days is my standard. Sometimes that means pushing a blade a couple of days past its prime, sometimes it means tossing a perfectly good blade that could go several more days, but it's convenient to change blades on Sunday when I have time to give the razor a cleaning. But if a blade has a problem, or just isn't cutting it any more, then it gets tossed immediately.
I generally change out a blade on Sunday for my face shaving razor but may push a little farther if my Saturday shave was still smooth. I've started a small weekly rotation of razors, so each week the razor change means a new blade. The three razors I am using on my face are the Blackbird, a Fatboy and a NEW LC. My head shaving razor is a Weber and it generally gets a new blade after 3 full head shaves, but I am using a Feather that just finished shave #5, so I guess I have to throw that idea out the window.
