Disc brake rotor wear limit
You will need to measure the thickness of your rotor to determine if it needs replacing, then check that against the manufacturer's recommendation for minimum thickness. I think your rotors look like Sram ones,  but I may be wrong, so I'll put the specs for other brands as well. Sram rotors should be replaced by the time they get worn down to 1.5mm thick. For Shimano rotors, it is 1.6mm. Magura recommends replacing at 1.7-1.8mm depending on the model of rotor. Hope have a limit of 1.4mm. Hayes should be replaced at 1.52mm.