It depends on the kid, when they need to wake up, and how much of a mess they are in the mornings.
My 14 year old has never slept much, and I learned pretty early that enforcing bedtimes wasn't going to work. If she's awake after 9:30 on school nights or up before 6:00 on school days, she's expected to read, do homework, or do something other than electronics. As far as I can tell, she falls asleep around 11:00, is up around 5:00, and doesn't seem to need more sleep.
The 11 year old has to wake up at 6:30, and tends to be a complete mess if I don't force an 8:30 reading quietly/9:00 lights out time.