When does a default Analytics session expire?
- At noon every day
- When a user is inactive on a web page for more than 30 minutes
- When a user opens another browser window
- After 30 minutes, regardless of user activity on your website
In Google Analytics, session is the period of time a user is active on your site. Note, that by default, if a user is inactive for 30 minutes or more, any future activity is attributed to a new session. However, you can adjust this setting. Users that leave your site and return within the expiration time limit are counted as part of the original session.
Sessions and campaigns end after a specific amount of time passes. By default, sessions end after 30 minutes of inactivity and campaigns end after six months. You can change the settings so sessions and campaigns end after the specified amount of time has passed.
Read more here: https://support.google.com/analytics/answer/2795871