Check out the Oracle® Enterprise Manager Ops Center Security Guide under "Protect Session Data" .
You need to modify /var/opt/sun/xvm/bui/webapps/emoc/WEB-INF/web.xml
<!-- normal and config flow session idle timeouts, in minutes -->
Although the guide says about using a smaller value increasing it works as well =)
We found that session timeout value will prevent an auto logout (you don't get dumped to the login screen after X time) if its extended, but if you don't pull data (I usually do a shift-reload) before trying some actions, you can sometimes still be logged out. For some reason shift-reload resets its idea of your activity timer without forcing another login.