How long stays my points valid with Marriott?
24 months, but within 24 months before they expire, you can purchase 1,000 points for $12.50 (or stay at a Marriott, use a Marriott Rewards credit card for at least a $1 purchase, etc) to keep them active.
Points don't expire ... unless you have no "qualifying activity" for 2 years, in which case they do expire at the end of the 2 years.
Qualifying activities include the following. Whenever you do any of them it resets your 2-year timer to zero:
For more information see Read Marriott Rewards Points Expiration Guidelines
Retrieving data ...