Average time on site is the average duration that people have viewed on your site. A high bounce rate means a low average time on site – users are not staying enough to take the desired action for achieving the business goal. Also known Session Duration in Google Analytics, it is calculated by dividing the total duration of all sessions (in seconds) by the number of sessions.
Session in Google Analytics is simply a visit to your website. A session starts when someone views a page on your website and ends either when they leave your website or after 30 minutes of inactivity. Everything a user does during that visit is counted as a single session.
Here’s how to calculate average session duration:
Total duration of all sessions (in seconds) / The number of sessions