Average time on site is the average duration of time spent viewing your site. A high bounce rate means a low average time spent on site – users are not staying long enough to take the desired action for achieving your business goals. Also known as Session Duration in Google Analytics, it is calculated by dividing the total duration of all sessions (in seconds) by the number of sessions.
A Session in Google Analytics is simply a visit to your website. A session starts when someone views a page on your website and it 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 the average session duration:
Total duration of all sessions (in seconds) / The number of sessions