Session length is typically used in terms of app usage. It is the amount of time from when a user lauches teh app until they stop using or leave the app. It tracks the duration of an individual, continuous experience in the app. Depending on the type of app, Session Length can be defined more narrowly as needed (e.g. a streaming app may timeout after a certain duration or track time running in the background). [1]
For a good average session duration, the industry standard is 2 - 3 minutes. [2]
[1] Geckoboard. Retrieved May 21, 2021, from https://www.geckoboard.com/best-practice/kpi-examples/session-length/
[2] Retrieved May 21, 2021, from https://www.spinutech.com/digital-marketing/analytics/analysis/7-website-analytics-that-matter-most/