Chapter 3 Authentication for FortiOS 5.0 : Configuring authenticated access : Authentication timeout : Security authentication timeout
  
Security authentication timeout
You set the security user authentication timeout to control how long an authenticated connection can be idle before the user must authenticate again. The maximum timeout is 480 minutes (8 hours).
To set the security authentication timeout - web-based manager
1. Go to User & Device > Authentication > Settings.
2. Enter the Authentication Timeout value in minutes.
The default authentication timeout is 5 minutes.
3. Select Apply.