Hello
Just want to know, plus minus how many km can a phantom 4 motor go on for, before replacing it to be safe.
Thanks

no set mileage, brush-less motors have an unlimited life span if well maintained. Usually bearings fail, dirt gets into motor, etc.
Your biggest worry would be the ESC blowing a capacitor or wires coming loose, but again thats quite unlikely.

I usually blow out my P4 motors every few flights with compressed air from my shop compressor at high psi. Motor bearings are sealed so no chance of blowing out oil, just dirt.

Cheers