Generally speaking, coins can be used at any time, depending on how you wish to use them. For example, certain Power-ups (coming soon) can be purchased ahead of time, but only used for in-progress quests. Once a quest is completed, you are unable to use any Power-ups to lower your time. It is recommended to plan ahead of time how and when you wish to use your coins. It is also recommended to complete as many Daily Challenges and Side Quests as possible to maximize how many coins you can earn.