Hello Chacko,
it would be nice to change the session timeout! But maybe the following workaround can help you:
You can reach the NAC server.log via NAC Manager (the Java tool) with a right click on you appliance and selecting "NAC Appliance Log..."
Or you use a SSH session to your NAC Appliance and browse your logs (there are several) in /var/log/ and /usr/local/Extreme_Networks/NetSight/appdata/logs
Regards
Stephan