When does everyone change to another razor? After every blade change or each day for example.
I generally use up a blade in one razor before moving on to the next. Not totally consistent about that, however. My injector blades last longer than the DE ones, so I might do a couple of shaves with an injector, go back to DE, then back to injector.
It's actually kind of a dumb side effect of me owning more razors than I have any need for.