I change out about every 3 shaves. The only difficult thing is I have 3 razors in rotation and remembering what I changed last is somewhat of a pain. However, the cost is so low I don't sweat it.
That's why I move the blade from razor to razor every day, rather than trying to remember how many shaves I have on this blade or that blade.
Most blades for me are 3 shaves and out, a few might go 4, some only 2. The ones that only do 2 do not get ordered in bulk. Once ordered in bulk, the cost becomes a non-consideration since the cost is much lower. I generally do 3 passes + touch up each shave. Given a routine, after a while one gets to know when a blade starts to tug, and a blade brand's expected life on one's beard. Once I put a blade in a razor, it stays there until its 3 shaves are done. In that time I move another razor "on deck," where my mental energies mostly are focused. Works for me.
Last night I had Shave No. 8 on a Iridium Super. I think it's finally about done, but I'll try No. 9 tonight.
Depends on the blade and razor. For most DE blades 3, sometimes 4 shaves. I have pushed a carbon SE blade to 13 shaves.
I've had in the past (during my trial & error phase) blades which needed to be replaced after the 3rd shave or so but my main blade of choice, the feather, gets to be loved and used for 5 shaves (usually 2 passes each) before a new one is loaded. I have been trying now the Voskhod (still on blade #1) and it seems that one will also last 5 shaves.
