To begin open the Interactivity of your design.
Select the More Information module followed by the Automation icon (the little wand).

Complete the automation as follows:
Event - select 'When Popup is Shown'.
Delay - enter the length of time that you would like the pop-up box to display for. For example, 10 seconds.
Action - select 'Close This Popup'.

The above example shows that the popup box will display for 10 seconds before automatically closing.