I just called the Marriott Rewards line today and got signed up for the Platinum Rewards challenge. Nine stays in three months (which I'm doing through my business travel). Just wondering how long it will take for that promo info to show up in my account, under the My Promotions tab?
The other thing you can do is call customer service as soon as your 9th stay is credited to your account (if you want to keep track yourself). They will verify that you met the requiements and update your status which will be reflected online within an hour.
If I'm not mistaken, unlike the Taste of Gold bonus, you won't get temporary Platinum status while you work towards completion (at least that's how it worked for me).