Considering the price of blades, 3 times is a policy I have for DE shaving as my tough beard prevents me from getting much more shaves. However, I had blades last for as many as 7 shaves but why endure the lack of smoothness?
I can push it to four or five, but I usually change blades after three shaves. They're cheap and it's not like you have to wring the most out of a $4 cartridge.
With Feathers and my medium heavy beard and three pass shaves I usually get four really god shaves and can go to five. It's a bargin any way you look at it !!!
I have a few really low quality blades that I will shave once with, then toss. If I have a really nice blade that I want to use for a while, I'll use one of the cheapies to make my first pass, then follow with the nicer blade.
But most get 3-4 shaves before tossing. However, I can get 15 with a Personna 74.