I believe it's 8-10. But really, that's almost impossible.
With high schools starting at 8am, most kids have to get up around 5:30-6:00 to get there on time. That would mean falling asleep between 8pm and 10pm.
With their circadian rhythms changing to late morning to late night - it's contrary to what we make them do. And with activities and hours of homework?
My daughter does well to get to bed at 11pm most week nights, but it's surely not enough for her.