The average session duration in seconds during the specified period.
Avg. session dur. = total duration of all sessions / number of sessions (rounded down).
The session is a time interval during which the user has not interrupted interaction with the application. For mobile apps, the session starts with the launch and stops when the application has spent more than 30 seconds in the background. For web apps, the session starts with page view and stops when more than 30 minutes have passed since the last view.