Formula
Sum of session durations / Number of sessions
How to measure
Track session start and end timestamps for each user visit and average the duration across all sessions in the period.
Why track it
Session length is a useful engagement depth signal, though it should be read alongside frequency, since a longer session isn't always a better outcome.
Example
A learning app's average session length grows from 4 to 11 minutes after adding progress tracking that encourages users to finish a full lesson.