Just made a points booking here. This is the Cancellation Policy:
Cancelling Your Reservation- Please note that we will assess a fee if you must cancel after this deadline. If you have made a prepayment, we will retain all or part of your prepayment. If not, we will charge your credit card.
But there is no deadline mentioned!
Edit: The "deadline" shows up on the confirmation email even though it was nowhere to be seen on the website during the booking process. Chalk it up to Marriott's broken IT.