Avg. Visit Duration shows you the average amount of time visitors spend on a website within a session.
SimilarWeb calculates Avg. Visit Duration based on the time elapsed between the first and last pageview per visit (visits are closed after 30 minutes of inactivity).
Using this metric
For most websites, the longer the duration, the better. Use this metric to understand how likely are users to complete the website goal (page views, purchase etc.).