I read a comment from a member here which was something like "If I go to a razor and there is a blade in it, it's dumped after that shave." It makes for a handy way to remember especially if you use more than one razor.
Anyway, this blade changing issue seems to arise when someone has a fixed notion that a blade should last a week or so, and in fact for them the blade may be past its prime after only two or three or four days due to personal factors. In that case the remaining days are subsequently less than satisfactory, as blade longevity for them doesn't match their expectation or what some other guys report.
Guys here will get 2 to 6 shaves per blade, sometimes more, all depending on personal factors to them. The average is probably 3 ~ 4 shaves or so. Factors are the brand of blade itself, lubrication, sensitivity of their skin, toughness and density of their beard/whiskers, type of razor, number of passes per shave, personal technique, and so on. Only the user can determine when swapping in a fresh blade is needed.
As to when to change blades; at some point, the blade will become 'dull' in that it will not reliably 'shave' the whiskers. 'dull' is relative, because a 'dull' blade is still very, very sharp and can easily cut you.
But the blade will only 'cut' --not 'shave/shear' the whiskers. You will become aware shaving takes somewhat more effort; that the blade is leaving some stubble/uncut whiskers behind after a pass; that there is a little additional 'tugging' or 'pulling' that was not present the day before. So be attentive and change blades accordingly.
The routine I've fallen into is 3 to 4 shaves per blade --that's two blades per week. I don't try to extend this, though I can get two weeks or more of great shaves with the old Personna*74's. That said, you should always shave with a blade that is in its' prime.
I use Astra SP's, Derby, and Super-Max Platinums in a Slim set at 4. Generally three passes and cleanup.How many shaves are you getting before switching to a new blade? I expect a lot of variation here, as there are many different blades and hair types.