Challenge rewards are randomly generated every ~12 hours. The basic rule is: The longer the challenge the higher the reward. 

A short challenge may earn just some coins. A medium long challenge may earn coins and a normal donut. Long to very long challenges may earn coins, gems and a rare or epic donut.

Reward donuts are randomly chosen from the users unlocked donut recipes from the „normal“ and „goals“ donut category. If a challenge is reached and notifications are turned on, the user will receive a notification. The session does not stop automatically and must be stoped by the user.