Although there is an official policy of expiration after 24 months, I dont believe they have ever invoked the rule. I havent earned poins for several years, but happily spent some on a hotel night last weekend, and it was a doddle to do! Unliike my Hilton or Aer Lingus points, which disappeared without trace after a year!
Also, with the current Marriott Rewards Premier Visa Card offer on the website, you will automatically get your Silver Elite Level every year, a free night's stay every year, and 50,000 points right away (also good for some free staying). Very nice way to get a nice jump on things!