cl_timeout Variable Max is 3500
Inactivity time before a client is disconnected. Does not affect alt-tabbed timing out any longer.
So if you set cl_timeout to 0 you most likely will disconnect the second you stop doing something in game. If it is at 0 you also will be unable to connect to any server as this is also the variable for how long you have to connect to servers. CAL default is 35 which is sufficient time.