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 session-config as well as other settings defined there in your web.xml.
This is actually from my Tomcat 7 (Windows) but I think 5.5 conf is not very different