Rating is League dependent, so your League rating is set to 0 every time you enter a new League (except when entering Elite League).
Fun Run 2 Leagues reset every week on Wednesday and all players' League score is set to 0. Prizes are also delivered on Wednesdays. You will either stay in your current League or be demoted one League down depending on how you place.
You can see your global rating in the Hall of Fame under the "crown" tab in the ratings screen. This rating never resets.