Here's the requirements in a nutshell.
Originally Posted by Marriott Customer Service
Marriott Rewards also offers Emeritus / Lifetime Status for members having 12+ years as a member, 600, 800, and 1,000 personal nights,
1,500,000, 2,000,000 and 3,000,000 lifetime points and 1 earned year at Silver, Gold and Platinum elite level in order to reach Lifetime status in these three categories.