I use my blades once Perfect shave every time. 2 days off then 1 shaving session, two days off then again. my skin loves it.
I get 5 shaves from my Derbys and 5 from my Bluebirds. I cork them before using them but all 5 of those shaves are perfect. I only throw them out because I don't want to risk having a bad shave. However, I feel they could last longer.
+1whatever blade i use gets canned after 4 shaves. Works for me, and of course ymmv.