I have the personal Priority card and I assume it is the same. It is per your use year. It should reset on the statement after your anniversary. I opened my card last October 25th. My AF was just charged Nov 1. This billing cycle ends November 14. So my upgraded boarding should reset on November 15. I SMed them and that is how they explained it. I was hoping for actual dates specific to me, but they gave me generic wording.
*"Annual" means the year beginning with your account enrollment date through the first statement date after your account enrollment date anniversary, and the 12 monthly billing cycles after that each year.