You can estimate the session duration by taking the last session heartbeat (which is taken every 5 seconds) and subtracting the start message time.

On that note, you will see cases in the dashboard where OOMs are measured as just a few milliseconds. In these cases a start message has been fired and a heartbeat milliseconds later, but the app OOMs before the next heartbeat at 5 seconds is recorded. This also explains why OOM durations are generally found in multiples of 5 seconds.