How long do major rental car companies keep their cars before selling them?

Does anyone know the standard for how long a rental car company such as enterprise, hertz or avis keeps their units before selling them? I've noticed throughout my travels that none seem to be more than a year or two old, and none have over 30k miles.How long do major rental car companies keep their cars before selling them?
They will usually get rid of them between 25,000 to 30,000 miles, just before the manufacturers warranty is up................How long do major rental car companies keep their cars before selling them?
Usually about 1-2 years at the most before they sell.