your JBossinsallHd:\jboss\server\default\deploy\jbossweb-tomcat.sar\web.xml

<session-config>
<session-timeout>30</session-timeout>
</session-config>

I believe \server\default\deploy\jbossweb-tomcat.sar\web.xml
will define it for all webapps. So, define application-specific parameters in the applications WEB-INF directory of the web application.

<!-- This document defines default values for *all* web applications -->
<!-- loaded into this instance of Tomcat. As each application is -->
<!-- deployed, this file is processed, followed by the -->
<!-- "/WEB-INF/web.xml" deployment descriptor from your own -->
<!-- applications. -->
<!-- -->
<!-- WARNING: Do not configure application-specific resources here! -->
<!-- They should go in the "/WEB-INF/web.xml" file in your application. -->