Chapter 10 Install and System Administration for FortiOS 5.0 : Using the web-based manager : Basic configurations : Changing the web‑based manager idle timeout
  
Changing the web‑based manager idle timeout
By default, the web‑based manager disconnects administrative sessions if no activity occurs for five minutes. This prevents someone from using the web‑based manager if the management PC is left unattended.
To change the idle timeout
1. Go to System > Admin > Settings.
2. In the Administration Settings section, enter the time in minutes in the Idle Timeout field
3. Select Apply.