It is suppose to start as soon as you reach that level and continue for the next calendar year. However, they are the worse when it comes to record-keeping. Remember, last year I lost my status even though I had rented for 55 days. It took several emails and a apology letter from the company and I finally got my status back. Had I not stayed on top of it, it would've never gotten fixed. I now print my status emails (hopefully you got yours last week?). You have to watch them when counting too. Sometimes they only count nights and you don't always get credit for part days. Early on I missed the status by 1 day. I had counted 44 days, but they only counted 39, guess who was considered right? It wasn't me, so I had to downgrade the entire year (had I known, I certainly would've added a day to a trip, or rented locally for a few days). I watch like a hawk now. I'm at 29 days now, but have a 6 night trip in Sept/Oct and a 12 night trip in Oct/Nov, so I'm good. As soon as you reach your required 40 days, shot them an email and ask about the upgrade!
BTW -- when you reach your status this year, you should stay at that level until February, 2012.