Did you check the documentation at
http://e-docs.bea.com ?
I searched for "session timeout" in the wls 8.1 documentation:
Using Sessions and Session Persistence in Web Applications from that URL (formatting better on their page),
"Session Timeout
You can specify an interval of time after which HTTP sessions expire. When a session expires, all data stored in the session is discarded. You can set the interval in either web.xml or weblogic.xml:
Set the TimeoutSecs attribute in the session-descriptor element of the WebLogic-specific deployment descriptor, weblogic.xml. This value is set in seconds. For more information, see session-descriptor.
Set the session-timeout element in the Web Application deployment descriptor, web.xml. For more information, see session-config. "
Two links in that blurb point to:
session-config session-descriptor