I believe you've got 5 years before you would need to retake the initial exam, ie. you could get your medical on 1st Jan 2008, then do nothing, and be able to go for a class 1 renewal on 31st December 2012. That's what they told me when I got mine. Coincidentily that's the same length of time that my associated Class 2 privileges are valid for, but I don't think the 2 timeframes are linked.