I think this might even be easier for you.
Set the maximum idle time for a session

Unless you just want to be notified and not actually end the session. But it would be good practice to set this in Active Directory via the user account if for nothing else, the networks' safety.

Or in a larger environment you could roll this out via a GPO.

Legacy Forum Name: Monitor Sets,
Legacy Posted By Username: TITaN

If it helps there are actually command line terminal services tools. The one you would want is called query.exe. You could query your terminal server and then dump the info into a text file and somehow set an alert on that.

http://technet.microsoft.com/en-us/library/cc776289.aspx?ppud=4
has some more information about the tools.

Legacy Forum Name: Monitor Sets,
Legacy Posted By Username: CBHighland