I think it depends upon the kid. My son, 15 1/2 (10th grade) stopped having a set bedtime a few years ago. Why? Not because he didn't need to go to bed, but because he started going to bed on his own (around the same time I'd always set bedtime). Now, he just randomly gets up from whatever he's doing around 9:30 on average (sometimes it's 10:00 sometimes its as early at 8:30) and announces he is going to bed. He makes the rounds and gives goodnight hugs (me, Dad and the dog) and off he goes. I know he doesn't go right to sleep... I can hear noises as he tosses and turns or fiddles with something or blows his nose (he has allergies), off an on... But he isn't on his phone--he puts that in the kitchen as part of his going to bed routine.
Our daughter is only 12. She doesn't have a "set" bedtime anymore, and she has a tendency to stay up later than is probably prudent sometimes. Usually because she is reading something. Or messing around on her keyboard (piano). Rarely because of TV/video, etc. Generally, she goes to bed around 10:30. I do have to tell her to go to bed sometimes (put the book away, etc)... but most of the time I can mention that it is starting to get late, or that it is nearing 10:00pm and she'll wrap up whatever and head off to bed shortly thereafter.
On the weekends, she doesn't really have a bedtime, and last night fell asleep in our bedroom floor around 11:00 where she'd been watching hockey. Son was long since gone to bed, since was up at 6:00 a.m. for wrestling on Saturday morning. He also went to bed early on Friday night, because he knew he had to be up on Saturday morning.
So, in response to the question, I would say that you stop giving it when they become mature enough and responsible enough to go to bed when they need to... whatever that means. My kids wake up at 6:05 to get ready for school since they have to catch the bus by 6:48 a.m. They need "x" hours of sleep to function well. If they were staying up too late, then I'd be giving them a bedtime still. They really need 8 hours sleep, but sometimes they need more. Sometimes, they are ok with the odd night that they only get 7 hours. They know themselves and what they can handle. But if your daughter doesn't... then you have to help her until she does.
At my son's age (when I was in 10th grad), I remember staying up until 1:00 a.m. reading, or finishing homework projects that I had neglected until the last day before it was due. (I always made As though!). Sure, I was tired the next day, but my grades didn't suffer. And I didn't stay up like that every night. I also didn't get up at 6:05 in the morning b/c we lived right across the road from the school--it was more like 7:05 a.m. when I woke up. I've always been a night owl (as I sit here at 11:19pm typing)... but I could handle it. Some kids can't. Their bodies aren't wired that way.