I was told that the fleet typically gets turned over twice a year. Spring and Fall/winter.

The Crossfire is a special case where they could not sell the cars at retail. Chrysler even tried blowing them out on Overstock.com at one point. After that they stopped production for the 2006 model year, since they didn't not sell all their 2005 models. Then finally dumped a lot of them to fleet sales we're seeing now.