When we are on our normal routine all my kids go to bed at the same time between 6 and 7 (depending on supper... obviously on nights they have activities it might be later)... they are pretty well asleep in about 30 minutes (I read for 15-20 minutes, Evie always falls asleep during this, the others sometimes do or fall asleep shortly after I leave the room) They get up around 7am. So all four of my big girls get roughly 12 hours of sleep.
However if they do go to bed later, they don't sleep later... that is why we get them in bed so early. AWANA night we don't get home until after 8, they are asleep by 9, and still get up at 7.... so they are getting at least 2 hours less sleep that night than normal. Oh, and none of mine (except the baby) nap.
One reason I homeschool is because I do not feel that kids get enough sleep nowadays. My kids are by no means lazy, and absolutely will NOT lay in bed

if they are awake. So I feel getting them in bed early for them to get the amount of sleep they seem to need is a good idea.
Also, my oldest has OCD which is drastically compounded when she is even the slightest bit sleep deprived. I've heard that hormones kick the OCD up a notch, so I will be insisting on her getting a massive amount of sleep when she starts going through puberty.