I think it would depend on the child and the level of activity they have during the day. Sports and physical activity can increase the amount of rest you need.
My daughter who is 11 usually goes to bed by 10 pm. She wakes up in the morning at 7 and 9 hours is usually what she needs to rest. Even during the weekends, she usually sleeps a total of 9 hours per night.
My teenager (17 yrs) goes to bed around midnight and gets up at 6, but takes a 2-3 hour nap when he gets home from school at 2:30. He says he prefers it this way and feels more rested.