In reply to Loughan:
I always try to jump off on the very first belay with a beginner. Obviously make sure they are backed up by a competent friend first.
Then then know, from the first, that they can stop a fall.
If you cannot have a experienced friend or passerby holding the dead end ready to lock off then you really, really, shouldn't be getting the beginner to belay. It's not a "ideally" in my book, its a "necessity".