Keep using the cutter till it snaps then replace unless it leaves keys rough and not clean enough to slide in smoothly. if you have machine set on a higher speed it’ll just snap it.
Change probe when it breaks. Change cutter when it breaks or starts miscutting keys.
No… Really, only change them when they break. I’ve only broken 1 probe and that was from a dirty key I didn’t clean before putting in the clamp! I’m still on my 1.5mm cutter from over a year ago.