I know this sounds crazy but I change blades once a month, on the first. I use HB triples. In fact, I get a few razor bumps the first time I use a new blade. Usually by the second or third use, no bumps. I am still getting a close shave on day 30. Of course, I am MPB and have very fine hair. I shave in the shower and simply wash the blades with the pressure of the water. Sure saves on blade expense.
I think it depends on which blade you use. I find that with the HB triples I get many more shaves than other blades. I don't know what it is. And I recall Timmay saying he used one for about six weeks or so.
With the (HB)doubles, I can get a few more shaves than with the Atra. I think it comes down to a few things together, shaving frequency, coarse or fine hair, blade quality and probably prep and blade care factor in too.
But I think most guys average about 7-10 days with a cartridge.