What is the lowest thickness my Rear Rotor can be?

Rear Brake Disc
Minimum Thickness. 0.433" in. ( 11.00 mm. )

Front Brake Disc
Minimum Thickness. 0.98" in. ( 25 mm. )
Runout. 0.0005" in.
Thickness Variation. 0.0002" in.

Well, when inspecting them a dealer may say, you need to replace them with the fronts rotors around 75K -80K miles, but that is not really true. They may be a little warped but mechanically they are not compromising the integrity of the braking system.
They were a little warped because I let my pads wear out and began grinding metal on metal before I changed my brake pads and pads wear unevenly sometimes. So I simply had them turned.

When finally replacing my rear rotors, I did not go by thickness, I went by the look and the amount of grooves in them made by my low brake pads since turning them initially. I had deep groove and since they were turned or ground before, they could only be turned so much more. For liability a dealer will say they are only allowed to grind them so much, which I believe is true. While I have some dire hard friends who have never replaced their rear rotors, I believe there is a point of pushing the envelope for proper brake functionality. It will provide less surface contract for the pad to touch when applied.

So anyway, long story short, look at them. If you have never replaced them before and are pushing high mileage, a change may be in order. But if you do not see any deep groove, they probably don't need it regardless of mileage.

If you do change them, or any other part on your car, go to the autozone and buy the parts and take them to the mechanic for labor only. This way, when you need to replace your brakes/front rotors again, they will be free. Autozone offers lifetime warranties on may parts. Plus if you have a mechanic do it, that is where they go anyway.

I think once you replace the rear rotors, you will not have to replace them again for the life of the vehicle. But I definitely chose to have mine replaced since I plan to drive my vehicle successfully well into her 250K mile or more.

Hope this helps. This is only my opinion.