Do the Harry Potter books discuss whether the Hogwarts students have a specific curfew? If so, do the books identify what the curfew is?
9.00pm - 10.00pm, seemingly dependent on the age of the student and possibly time of year.
In Harry Potter and the Order of the Phoenix we're told that curfew for fifth years is 9pm
At half past seven Harry, Ron and Hermione left the Gryffindor common room, Harry clutching a certain piece of aged parchment in his hand. Fifth-years were allowed to be out in the corridors until nine o’clock, but all three of them kept looking around nervously as they made their way along the seventh floor.
Harry Potter and the Order of the Phoenix
However, in Harry Potter and the Goblet of Fire, we see Harry out at approximately 10pm without fear of breaking curfew.
We are advised that it is June.
But Ron’s tolerance was not to be tested much as they moved into June, for Harry and Ginny’s time together was becoming increasingly restricted.
Harry Potter and the Half-Blood Prince
Harry is called to Dumbledore's office and the narrator advises us that curfew starts shortly.
Once Peeves had vanished, there was silence in the corridors; with only fifteen minutes left until curfew, most people had already returned to their common rooms.
About ten minutes later (after having had a short conversation with Professor Trelawney) Harry heads to Dumbledore's room and is told to meet him at the stairs in a further five minutes time, which he does, presumably just as curfew is being called. Since we know the the sun hadn't quite set, this means that curfew has to be a few minutes before sundown, which in June in Scotland would be between 9.50 and 10.05pm
‘Very good. Then I wish you to go and fetch your Cloak and meet me in the Entrance Hall in five minutes’ time.’ Dumbledore turned back to look out of the fiery window; the sun was now a ruby-red glare along the horizon. Harry walked quickly from the office and down the spiral staircase. His mind was oddly clear all of a sudden. He knew what to do.
We know it is at least after 11:00 pm for 6th years from this scene:
Bewildered, Harry looked around as a small golden clock standing upon Slughorn’s desk chimed eleven o’clock.
“Look sharp, Tom,” said Slughorn, turning around and finding him still present. “You don’t want to be caught out of bed out of hours, and you a prefect…”
Since Slughorn is concerned about Tom being caught out of bed, this suggests curfew is not long after 11.