I have a wide range… I have many that stay on for over 10 minutes on up to keeping the pages up for days. However, from what I can tell, if the user does not click beyond the entry page, it gets tracked as 0. They might be there for 5 minutes, but since they never move pages I think it doesn’t know how long.
Not sure if this helps you or not. I think everyone will be different.
If I were to average it, it would be very skewed due to the people leaving up my pages for hours and days.