A sliding scale, in my case. When I first had to shave daily for work, not quite 50 years ago, I was young and my whiskers were too. Most blades seemed to last me from Monday to Friday, and perhaps Saturday, and, as they were relatively expensive on my pay, I eked them out anyway. As the old bristles hardened up, the blades ran out sooner. I also had sideburns, moustaches and "Piccadilly Weepers" at various times, and of course that reduced the area needing to be mowed, thus slightly increasing blade life. The other guaranteed method of ensuring they lasted was to grow a full beard; but I digress.
I've got a few 70's vintage blades, which I occasionally use, for old times' sake, but they don't perform much differently to my modern ones (the decent ones, that is, and not things like Zazas), and get the heave-ho after 3 days, one way or t'other.