This is mostly based on the wear limit which will be stamped on the rotor. Rotors that begin at 1.8mm, typically have a wear-limit of 1.5mm.
For example, SRAM rotors are now 2.0mm thick as of the HS2 rotor released in 2022. This has a limit of 1.7mm.
But it is NOT all about the thickness of the rotor. Often times rotors wear in unconventional ways long before their wear limit; potentially creating a concave surface (which then can create stripes on the brake pads which you can see here).
There is no concrete answer with the exception of the wear limit. If you installed new brake pads are are getting noise, you might be able to follow our Rotor Reset Process to get the rotor back to zero which typically resolves the noise issue.