Chapter 16 SSL VPN for FortiOS 5.0 : Basic Configuration : User accounts and groups : Authentication of remote users : Setting the client authentication timeout
  
Setting the client authentication timeout
The client authentication timeout controls how long an authenticated user will remain connected. When this time expires, the system forces the remote client to authenticate again. As with the idle timeout, a shorter period of time is more secure. The default value is 28800 seconds (8 hours). You can only modify this timeout value in the CLI.
For example, to change the authentication timeout to 18 000 seconds, enter the following commands:
config vpn ssl settings
set auth-timeout 18000
end
You can also set the idle timeout for the client, to define how long the user does not access the remote resources before they are logged out. For information see “SSL connection configuration”.
See Also
Authentication of remote users
Allow one time login per user
Strong authentication with security certificates