Threshold to scrap: if of the six natural rolls (i.e. before racial adjust etc.) nothing is higher than 13* or the average of all six is less than 10.0 you can choose at that point only between keeping it or starting over. If you go any further with char-gen you're committed to keeping it.
It's fairly uncommon that this threshold is met, but it happens. That said, one of the most successful characters in my current campaign started with an average of about 10.5, proof that there's hope even for the nearly-hopeless.
* - bonuses etc. from stats don't start until 15 in my game.