Hi CRIS,
This is not a simple question, it may take a while to locate and as you said often if it is the default value there is no "tweakable" registry key and I am supposing you are using PPTP for VPN client.
Since there is only one article about this timehold value, however it is very old. I had to dig it in registry and I think it is MaxConfigure
HKEY_LOCAL_MACHINE\System\CurrentControlset\services\RasMan\PPP
In this article it mentioned that to alter the default, the user must add the following registry parameters to the system.
https://support.microsoft.com/en-us/kb/193841/en-us
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Raspptpe\Parameters \Configuration\
InactivityIdleSeconds: REG_DWORD: default: 60 Number of seconds PPTP will go with no activity on the control channel.
EchoReplyTimeoutSeconds: REG_DWORD: default: 60 Number of seconds PPTP will wait for a reply from an echo request. If reply is not received, PPTP drops the connection.
I didnt find this value in Windows 7 and Windows 8, but I am supposing you could give it a try by create a new value in
HKEY_LOCAL_MACHINE\System\CurrentControlset\services\RasMan\Parameters
IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs.
For information on how to do this
https://support.microsoft.com/en-us/kb/322756
Regards,
D. Wu