The visit duration is the average time a visit stays on the website expressed in standard format (hh:mm:ss).
Visit duration is calculated as the total time spent by visitors divided by the number of visits with a registered visit time.
Notice that the last action in a visit has no time. This is because timestamps are only set at the beginning of each action. It follows from this that bounced visits have no registered time. They are therefore excluded from the calculation of visit duration.
The measured visit duration may be both shorter and longer than the real visit duration. It may be shorter because the actual duration of the last action is not taken into account. It may also be longer because visits with only one page view (bounced visits) probably spend less time on the website than other visits on the website.