First of all, sorry for the delay. Actually I've made a cron entry that simply calls a script that does the following query (note that the RDBMS is PostGreSQL, so I don't know if this query will run smoothly on any RDBMS):
DT = current_date AND
HR < current_time - interval '30 minutes'
This brings the ID's of the expired sessions (in this case those idle for 30 minutes or more), so I can delete them from the database. What happens next is that when the user decides to click another link or something like that, each page checks if the session is still on the database. Since it's not, the user is automatically redirected to the login page with a message telling him his session has expired.