When I was using cartridge razors, I got used to using dull blades. I got used to it because they were $5 a pop. And I was being both cheap and lazy. My expectation were also very low. Just get enough of this stubble off to look presentable. Good? Got it. Done. I have different expectations now. And because I can hone my razor, there is no reason to get used to a deteriorating edge. Five to ten passes on my finishing stone and I'm right back to optimal sharpness. How long? I have no idea. The minute I notice it starting to slip I fix it. I like a freshly honed edge. So I never let it get too far before it gets a couple laps on finishing hone. And the more razors you have, the harder it is to keep track of how long its been. I don't allow myself to get used to an edge that's past its prime.