the minimum thickness will vary according to the manufacturer of your discs, some are harder than others. there will be a wear indicator which will likely take the form of a groove or in some cases the makers name cut into the disc. when this starts to fade or disappears it's time to change. brakes are one area you do not want to cut costs on.
Front: Original Thickness- 28mm. Minimum Thickness- 26mm.
Rear: Original Thickness- 12mm. Minimum Thickness- 10mm.
Not much wear tolerance. I have 62k on my 2.2jtd and on 2nd set of pads front and rear. Front at about 46k and rear at 60k. Discs are originals and are still within tolerances. Front will need to be replaced at next pad change but I am not sure about rear!
