I got my PRG in late December and received a letter in April stating that my card was eligible for POTO enrollment. I still haven't activated it and don't know specifically what makes an account eligible but if I go to the activation page some of the information reads:
"Not all Card Members are eligible to use Pay Over Time features. Requests to pay over time are approved based on several factors including account history, current account status and credit record at time of request. Upon enrollment in the Extended Payment Option, we will assess your current Pay Over Time Limit, which may be set starting at $10,000.
The Annual Percentage Rate ("APR") for Pay Over Time features may vary and is determined by adding the Prime Rate plus 14.99%, currently 18.49%.
This offer to enroll remains valid as long as you are a Card Member in good standing and there has not been a material change to your credit worthiness"
edit: Apparently, my account is enrolled in POTO (Select & Pay Later) with a $35k limit and was done so, automatically, last week. The information quoted above is in regards to a feature of POTO called "Extended Payment Option" (basically, S&PL automated). However, I don't think that would change the list of things Amex would examine; history, account standing, etc.
No MR offer here 
I think they always start with 35k, mine also. But after a while I received a letter, telling me that it was increased to 50k. Even so I never ever used it.
