You can change the autodisconnect time on Dial Up Connections
through the registry.Navigate to :


- HKEY_LOCAL_MACHINE\System\CurrentControlSet
\Services\LanmanServer\Parameters


Locate here in the right pane the entry :

autodisconnect

Doubleclick it and set its value (in Decimal) to the minutes
you want to have it.


For example 15 means you get disconnected
after 15 minutes.