While every blade is different, I'll often change it after 4 shaves. Sometimes if the blade is feeling pretty good I'll stretch it to 5 shaves. But with the cost of blades its just not worth the risk of irritation to try for too many.
I get two uses out of every blade. The only brand i can push to three is the Gillette Sharp Edge, but that third shave has questionable quality. Blades are cheap enough, so no need to treat them like carts.
I have no hesitation about changing blades if I detect a problem, but usually change them the first shave of the work week. Then I get 5-7 shaves out of each one, depending on whether I stay at the apartment or head home for the weekend.