To maintain a Streak, the time between completing challenges must be less than 24 hours. You can always check the remaining time for the next challenge submission in the Streak menu.
If your Streak is broken, you can pay to repair it. After the Streak breaks, the game will offer you the chance to restore it. If you decline this option, your Streak will reset, and you’ll need to start over from the beginning.
You can also repair the Streak by tapping on the yellow floating button on the main game screen.