In Order of the Phoenix. Harry, Hermione, Ron, Luna, Ginny and Neville fly from Hogwarts to the Ministry of Magic on Thestrals. Hogwarts is in the Scottish Highlands, roughly 400 miles from London as the Thestral flies. How long did this take them and how quick do Thestrals fly?
I'm not going to be 100% accurate here, but Firebolt broomsticks are listed as having top speeds of at least 150 Mph. It's also said in OotP that Harry felt like he was going faster than he ever had before, and he was the proud owner of a Firebolt. Knowing these two things, we can say a Thestral has a top speed surpassing the Firebolt, meaning it likely goes at least 180 Mph for Harry to be able to notice the difference in speed.
Hogwarts' exact location in Scotland is unknown. Hermione says at one point that it's near Dufftown, in upper-eastern Scotland, but the landscapes don't really match up too well. We'll take her word for it though, and say that Hogwarts is roughly 580 miles from London.
Knowing that distance and a roughly correct speed for the Thestrals, we can easily calculate the times as being:
580 Miles / 180 Mph = 3.22222222 hours. If we take into account that my estimation of Thestral speed is shaky at best, and that my/Hermione's estimation of Hogwarts' exact location is equally so, we can easily extend the range of time taken to be 3-4 hours.
It's a very rough guesstimate, but without knowing more precise details like the actual speed of a Thestral or the exact location of Hogwarts, it's difficult to provide a more accurate timeframe.
As they headed into the Forbidden Forest, the sun was still partially up.
The sun was falling toward the tops of the trees in the Forbidden Forest now as Hermione marched purposefully across the grass, Umbridge jogging to keep up.
They walked for sometime into the forest, had the confrontation with the centaurs and the giant, and then got aboard the Thestrals. After they took off in the Thestrals for London, it became twilight.
Twilight fell: The sky turned to a light, dusky purple littered with tiny silver stars, and soon it was only the lights of Muggle towns that gave them any clue of how far from the ground they were or how very fast they were traveling.
They are clearly traveling faster than Harry's Firebolt, which supposedly can go up to 150mph. It is so fast in fact that Harry can't hear or even perceive much of anything, particularly with the wind chill.
He was deaf from the thundering in his ears and his mouth was dry and frozen from the rush of cold night air. He had lost all sense of how far they had come; all his faith was in the beast below him, still streaking purposefully through the night, barely flapping its wings as it sped ever onward...
So, faster than a Firebolt. How much? Impossible to say other than a setting sun and the twilight, there are no time references.
All quotes from Harry Potter and the Order of the Phoenix.
I always was amazed at how it took forever to go from King's Cross to Hogwarts or vice versa... They leave 11:00 AM and get there just before dinner. I mean I know it’s far and all, but they were able to travel on Threstrals to the Ministry in like no time at all. Though I do think that the trip was supposed to have taken at least 3-4 hours and that it was really late at night when the actual fight at the Ministry. That explains why there was literally no one in sight when Harry took the others to the Department of Mysteries. It was probably like 10 o'clock or later.