Hi Udit- Julianne from Mixpanel Support here.
Mixpanel automatically records a property called "Session Duration (Seconds)" with every "Session Start" and "Session End" event. You can perform aggregate calculations per user on this "Session Duration" property to find the average session time per user.
That said, the virtual session events are calculated based on event data being sent to your project. This means it would be unexpected for there just to be "Session Start" and "Session End" events. In order to calculate sessions, you would need to send event data. The Sessions doc has more information on how the virtual events are calculated and the properties available on them.Ā