I've probably done about 20k on the rotors I bought the bike with. They still work fine, but are visibly worn and slightly grooved from me not replacing the pads soon enough a few times.
How do I know when it's time to get new rotors (apart from by finding out the hard way, of course)?
Normally the manufacturer of the brakes gives some minimum value for the rotor thickness. For higher prized brakes they often even provide some gauge that lets you easily check if the rotor is still thick enough.
Typically the minimum thickness might be somewhere below 2mm (I believe to remember that the absolute minimum for Magura discs should be 1.7 or 1.8 mm). But as said, this should be an information that should be in the manual of your brakes.
Shimano recommends that its rotors, which start out 1.8mm thick, should be replaced when the braking surface has been reduced to 1.5mm. Credit https://road.cc/content/feature/when-should-you-get-new-disc-brake-rotors-257623
In my 14 years cycling career only once I had to replace rotor due to wear. And that rotor was 6 years old and used for heavy downhill riding in all conditions. I've gone through a lot of pads on that rotor (like 20-30 pairs). And only when I could actually feel with my fingers the groove on the surface, I replaced it. Also it started eating pads like mad - uneven breaking surface wore pads much quicker.
I'd say rotors is something you would not replace quite often. And rotors would not fail on you like a worn-out rims - rotors won't split easily. So for a sake of measurements, 30% thickness reduction would be a good indication to replace the poor thing.
