There is no published standard for getting PP status. Its not purely based on nights but is rumored to be a rolling average over 125nights + points/value of hotel stays and other things but basically the top 3% of all MR members is the rumor. Three's nothing that says "if you are 3yrs at 100 nights you'll get it" anywhere. I have over 650 nights in the past almost 4yrs and I just got PP status this year. My average nightly cost is over $100/night but typically under 150/night with rare exceptions.
If you can get the MR team to nail down an exact standard to achieve it, please post it up :)
I kind of wonder if part of the PP criteria has to do with lifetime accumulation of points, nights, and other stuff. I was Platinum for at least five years before reaching PP. One of those early years was 235 nights and the others were in the 130 – 180 nights a year. It may be a coincidence but I did not get PP until the year I hit 1,000 nights lifetime (now it’s 1,440). This is my 3rd year at PP and frequency of stays is about the same or less than previous years. My stays are about a 50-50% split between full service Marriott’s and Courtyards.