Platinum challenge means you get platinum benefits during the course of your challenge, so you have 3 months from the date of the challenge to make X stays, and if you make it, you get the status for the rest of the year.

And to answer your other question, I am not Gold with Marriott as most of my travel does not bring me to Marriott locations. Your point is well taken though, I did not check Marriott's site before making my Priceline offer, which I probably should have. I also did not know the offer would be accepted by Marriott anyways as there were a number of other 5* properties in the vicinity.