Storefront session timeouts are distinct from Business Manager timeouts and can be configured by in Business Manager with values in a range of 15-120 minutes. These changes are applied only to subsequent storefront sessions.
Per the Session Class documentation:
To change from the default session timeout setting of 30 minutes, please log into Business Manager and go to Administration > Sites > Manage Sites > [site] > Settings and enter in your desired value in the Session Timeout field.
Note: The hard timeout value cannot be changed.
For information on Business Manager sessions, please see Is It Possible to Change the Business Manager Session Timeout in B2C Commerce?
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