Windows 7 default TCP connect timeout

I'm thinking that you just need to connect from one machine to the other while running a packet capture on one of the machines. Make an FTP, HTTP, SSH, etc. session and just let it sit there until it times out.

I'm not sure what you mean when you say "In theory, I would send a packet with gradually increasing intervals until the connection is lost", but I don't think you need to do anything other than make a connection, capture the traffic, and let it sit until it times out. Timeouts occur on idle sessions and if you send data to the other end it will probably reset the timer as the session will no longer be idle.

When it does time out, look at the timestamp of the capture from the first packet (beginning of the three way handshake) until the connection is terminated (you may or may not see a RST).

Barring any application layer timeouts (depending on what type of connection you make) this should give you an idea of what the timeout setting is configured to.

In Windows the value is dynamic for established conections, though the default for initial connections is 72 seconds. The Registry settings are defined in this article:

http://technet.microsoft.com/en-us/library/cc739819(WS.10).aspx

HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services: \Tcpip \Parameters

TcpInitialRTT: Defines what the initial time-out settings are for new connections. This number in seconds is doubled each time it retransmits before timing a connection out. Defaults to 3.

TcpMaxConnectRetransmissions: Defines the number of retransmissions while establishing the connection before timing a connection out. Defaults to 2.