I presume you want a different page for timeout than login? If you just want to change the login page you can override that in configuration, have a look at this example: http://wiki.alfresco.com/wiki/Customising_The_Login_Page
If it is a different page you need, this is not possible currently out of the box. You can raise this is an new feature in JIRA if you want this considered for a feature release.
In the meantime, you'll have to take a look at org.alfresco.web.app.servlet.AuthenicationFilter and org.alfresco.web.app.servlet.BaseServlet. What you'll need to do is try and get a "LoginBean.LOGIN_REDIRECT_KEY" into the session after you detect a timeout. BaseServlet would then use that to redirect.
