Yeah, at least ten days for me too. Just to be sure! I candle on day 6 and mark the 'definites' and the 'maybes' and the 'definitely nots' with pencil on the shells. Then I put them ALL back in till day 12. I know I'll usually get a few 'maybes' developing and once or twice with thick and/or dark shelled eggs, I've also had the odd 'definitely not' go on to develop and hatch out a perfectly healthy chick.
I suppose if I got a better candling light I'd probably be able to tell for sure much earlier on, but I guess I like the element of surprise!
Also for whatever reason, I find it MUCH easier to pick out the 'definites' and the 'no ways' by day 6 if I'm incubating the eggs on their sides and they're being rolled side to side by the turner. When I'm using a bator that holds them upright and swings them back and forth, the veins and tiny embryo are always much less visible...