Reference:
The SSL VPN idle-timeout setting determines how long an SSL VPN session can be inactive before it is terminated. When an SSL VPN session becomes inactive (for example, if the user closes the VPN client or disconnects from the network), the session timer begins to count down. If the timer reaches the idle-timeout value before the user reconnects or sends any new traffic, the session will be terminated and the associated resources (such as VPN tunnels and virtual interfaces) will be deleted.