Dell SonicWALL GX250 Manual page 49

Note: If the Administrator Inactivity Timeout is extended beyond 5 minutes, you
should end every management session by clicking Logout to prevent unauthorized
access to the SonicWALL Web Management Interface.
Set the inactivity timeout by entering the desired number of minutes in the
Administrator Inactivity Timeout section and then click Update. The Inactivity
Timeout may range from 1 to 99 minutes. Once the SonicWALL has been updated, a
message confirming the update is displayed at the bottom of the browser window.
SonicWALL Internet Security Appliance Guide Page 49