Remaining time is shown on the far right, next to the clock.
If time runs out, you lose a life and will need to restart the given level. You can collect time bonuses to extend the time allotted for the given level.
When completing a level you are given bonus points for each second remaining.
If you selected Zen Mode when starting the game no clock will be displayed and you can take all the time you'd like to try to complete each level.