You can use a Install OnTimer Script to perform a script at a set time. I haven't used this script step to create a idle script, but I think it would be possible. You would have to reset the Timer every time a user clicks on an object or enters an object. You would have to issue a OnTimer Script step with no script to stop the current OnTimer Script then issue another OnTimer Script to restart the timer.
Thanks s chamblee... I managed to do this.