OK. Here's the latest as of the date of this posting. Preferred Select status is given--on request only--to those who have 9 rentals in a six month period. Apparently it doesn't matter how many days you rent, just the number of times. Also, the person I spoke with wasn't quite sure about this, but it appears that the six month period is a rolling window.

I was asked to call back 3-4 days after my ninth rental return. I'll update the post to let you know how I did.