Is there anyway to use the Session Data Manager or a related component to measure the length of a session? What I actually would like is an event that fires the moment a session expires and one that fires the moment a session is initiated (well, the last thing you could do by inserting something in the session and checking whether it already was there).

The session_data object (in the temp folder) has hooks for session start and stop. These hooks are also available through the API. Read the Zope Book (2nd edition -- on-line) about sessions for more info.