Setup > Security Settings > Sessions Settings
Timeout value: System log outs if the user is inactive after a length of time. The timeout is between 15 min to 12 hours.
Disable session timeout warning popup: System prompts inactive user with timeout warning message. User is prompted 30 sec before timeout as specified.
Force logout on session timeout: When enabled and if the user is inactive after specified time the current session becomes invalid. The browser will automatically refresh and will prompt user to login again into the Salesforce.