what is the wear limit (minimum thickness)...
what is the wear limit (minimum thickness) of the disc
I can't find an answer on this interweb thing. In my opinion, any noticeable difference in thickness between the braking surface and the original thickness of the rotor warrants a new rotor. You can usually get some years out of rotors if you replace your pads and aren't riding chairlifts daily.
Most cases of excessive wear I've seen in rotors are far from even across the braking service. The odd angles and grooves can eat through brake pads. It doesn't take many sets of pads to make a new rotor worthwhile.