When to destroy sessions that iterate

Prolly a silly question, but I have a session process that never really ends as it is supposed to allow the user to carry on submitting etc. So I don't know where and when to end the session, and if it is really nescessary to end a session? Obviously a logout button will do the trick, but what about aborting users that don't bother to logout? I presume a session will have a timeout value? If so, how does this work? Surely there can't be a physical time allowed for a session to stay alive? What about if I want to have an intranet application that could ( and preferably should) be able to be open indefinately? Ta