Your answer is here: http://www.rewards-insiders.marriott.com/thread/11973
There are three official Elite levels: Silver Elite, Gold Elite, and Platinum Elite, plus one unadvertised level above Platinum (Platinum Premier Elite, which is assigned by Marriott to the "top 3%" of Platinum Elites each year). Each of the three official levels has a corresponding Lifetime Elite: Lifetime Silver (250 nights/1.2 Million points), Lifetime Gold (500 nights/1.6 Million points), and Lifetime Platinum (750 nights/2 Million points). There is no Lifetime Platinum Premiere. The nights requirement is total creditable, and the points are total accrued, not "in the bank" at any one time.
Also delineated here: Elite Benefits
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