What is Average session duration?

Analytics starts a clock when someone arrives and stops it at their last recorded action. Add up all the visits, divide by the number of visits, and you get the average session duration.

The catch is the last page. If someone reads your final page for ten minutes and then closes the tab, most tools record no time for that page, because there is no next action to measure against. Long reads often look shorter than they were.

Read it as a direction, not a fact. Compare this month to last month on the same pages. A number that halves after a redesign is worth investigating. A twelve second difference is noise.