The List Acquisition and Refresh allows you to select how often to run the process to acquire and refresh email lists.

This process checks to see if there is a list that has been manually refreshed or a new list added under Email>Lists. 

The Last Run date/time stamp only shows the last time the a list was manually refreshed or a new list acquired. It does not show the incremental time frame of the lists being monitored for acquisition.

For example, if you have the List Acquisition and Refresh service to run every 20 minutes, your lists won't actually be refreshed every 20 minutes. This schedule simply checks to see if there is a list that has been manually refreshed or just added every 20 minutes. If there is a list that meets that criteria, then the list will be refreshed. Otherwise, lists are refreshed prior to sending a message and that is not reflected within the Last Run time.