Define inside web.xml

Open $CATALINA_BASE/conf/web.xml and find this:

<!-- ==================== Default Session Configuration ================= -->
<!-- You can set the default session timeout (in minutes) for all newly   -->
<!-- created sessions by modifying the value below.                       -->


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

All webapps implicitly inherit from this default web descriptor. You can override these default settings via your webapp's own web.xml file.

This is actually from my Tomcat 7 (Windows) but I think Tomcat 5.5's configuration is not very different