As tpasol noted, it's ten (for 5-Star) or forty (for PC) rentals in a calendar year (01/01-12/31). Officially, the status starts on January 1st of the year following qualification. So if you qualify in 2004, your status becomes effective on January 1st, 2005 and lasts through December 31, 2005.

However, call or write Gold Customer Service and they will activate it immediately, so you can "enjoy" the benefits with your next rental (I say "enjoy" somewhat tongue and cheek cause as a 5-Star, I have noted no differences from being a plain Gold. I might actually make PC this year, so we shall see what that gets me - at SEA, not much, I know, but perhaps at other stations I will get lucky).