I have seen Avis cars at 35,000+ miles, but never Hertz...at least not yet.
The HLE near me winds up with a lot of high mileage cars. They sometimes have to lie to the computer to let them rent me a car that is showing up as a "turn back," and that is usually at around 28,000 miles. The 2007 Fusion I have now just turned 25,000 so it will likely be yanked soon.