These should appear in 


HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<service name>\Config


TCPEnableKeepAlive Y

TCPKeepIdle 300

TCPKeepInterval 60

TCPKeepCount 8


Tthe values shown are the default values, those values are installed when you create a new service 
and they are also the values that we use if the entries are missing from the registry.

The AppxDSvc installer only writes a Config entry if it differs from the default value.  
That means that the installer won't write out entries for the TCPKeepXXX values, 
but AppxDSvc will pretend like those values are present (and set to their default values).

You can add the entries manually if, for some reason, you want to change the defaults.

Setting                 Argument        Default What it does
----------------------- --------------- ------- -----------------------------------------------
TCPEnableKeepAlive      true or false   true    Enable dead connection detection
TCPKeepIdle             (seconds)       300     Idle time before starting to send pings
TCPKeepInterval         (seconds)       60      Interval between keepalive pings
TCPKeepCount            (count)         8       Count of keepalive ping attempts before closing


The time before a server session closes would be the combination of all of these.  
Here is what happens.

After 300 seconds of idle time the server TCP stack will start trying to ping the client TCP stack.  
The server will make 8 attempts to ping the client and get a response before giving up and closing.  
It will wait for 60 seconds after each ping attempt for a reply before sending the next ping.

So in this case it will be 300 seconds + ( 8 * 60 seconds ) = 780 seconds = 13 minutes 
before the server session goes away.

appxd has environment variables for these settings as follows:

APPXD_DISABLE_KEEPALIVE set to anything will turn off the keepalive code.

APPXD_KEEPIDLE     = TCPKeepIdle
APPXD_KEEPINTERVAL = TCPKeepInterval
APPXD_KEEPCOUNT    = TCPKeepCount