I usually stop after 6. I have only used three blades: Wilkinson Sword (Black), Shark Super Stainless, and Derby Extra. In my opinion, the Derby Extras are the smoothest and last the longest.
YMMV but I don`t push my blades after the third shave, tho I might get 1-2 good shaves more. I preffer not to push my luck and where I live DE blades are dirt cheap anyway so its not a real waste and it is still cheapper then shaving with carts (a pack of polsilvers is 0.50 euro - lucky me )
For DEs I most often use Personna Med Preps, Feathers and Astras and on average get 4 three-passes shaves with the Med Preps and Astras, typically 3 for the Feathers. Since the number of shaves varies, I use a blade until the first sign of tugging, and then change it, even mid-shave.
I change each blade after 2 shaves, regardless of brand. I can, and have, pushed it as far as 7, but I see no reason to stretch blade longevity.
At an average cost of a dime per blade, its one of my cheapest luxuries. Even my most expensive blades, Feathers, cost me a quarter each, but even that is pretty economical.
The fact that I just discarded a Feather blade after 6 shaves is completely meaningless.
Your skin is different than mine, probably your razor as well, and also your technique.
Since I want a good quality comfortable shave more than I want to save a few pennies a week, I go two shaves regardless of the manufacturer. I have gone three but there is a noticeable difference in that third shave.
In my opinion the sharp blades like Feather tends to get dull sooner. Whereas Astra platinum lasts longer which is a smoother blade in my opinion, but less sharp than Feather. I think it also depends on how well you rinse and dry the razor after the shave.