When you add a new unit or edit an existing one, pick the "After a period of time" (1) option in the "How to complete it" field, and the following settings appear:

• Complete after (seconds) (2): Enter the amount of time (in seconds) that has to pass for the unit to be considered completed.
• Pause timer when idle for (seconds) (3): Enter the amount of time (in seconds) that has to pass without any user action for the timer to pause its countdown to completion. Leave blank when you don't want the countdown to pause.

For example, if you wanted the unit to be automatically completed in 15 minutes (900 seconds) and pause whenever the user has been idle for 15 seconds this is what you would type in the respective fields: