Install the Session Timeout plugin from the Bubble plugin marketplace. In the Design tab, add the Session Timeout element anywhere on the page.
Set the timeout property (in seconds) in the property editor (e.g., 60 for a 60-second timeout).
In the Workflow tab, create a new event "When Session Timeout → Inactive" to trigger actions when the user is inactive.
Add the "Log the user out" action under Account in the workflow to automatically log out the user.
(Optional) Create a "When Session Timeout → Active" event to trigger actions when the user becomes active again.