“Average session duration” is the total duration of all sessions (in seconds) divided by the number of sessions. You can use this metric to measure visitor quality. This metric is imported from your Google Analytics accounts and is calculated based only on sessions that originated from Google Ads clicks.
If you don't see data in this column, your accounts might not be linked properly or there might be no data for the data range you’ve selected (Google Analytics data is not available for dates before July 1, 2011).