How long is a lesson at Hogwarts?

Question

Over the course of the Harry Potter books we see quite a few lessons, but can we say for sure how long they are?

I've got a feeling this might get slightly messy. For example, in The Goblet of Fire Double Potions is an hour and a half long, which suggests Hogwarts has 45 minute periods:

Double Potions was always a horrible experience, but these days it was nothing short of torture. Being shut in a dungeon for an hour and a half with Snape and the Slytherins, all of whom seemed determined to punish Harry as much as possible for daring to become school champion, was about the most unpleasant thing Harry could imagine.

Harry Potter and the Goblet of Fire - pp.260-1 - Bloomsbury - Chapter 18, The Weighing of the Wands

And yet I get the very strong impression that lessons are actually one hour:

'The way he talks,' Harry muttered, as he hobbled out of the Defence Against the Dark Arts class an hour later (Moody had insisted on putting Harry through his paces four times in a row, until Harry could throw the curse off entirely), 'you'd think we were all going to be attacked any second.'

Harry Potter and the Goblet of Fire - p.205 - Bloomsbury - Chapter 15, Beauxbatons and Durmstrang

Unless we're supposed to understand that this class too was a double period and the events described just before this paragraph took half an hour and the "an hour later" refers to an hour from that point.

Answer 1

12 parsecs. No, really. It's JKR Maths. Therefore, anything numeric will be all over the place.

We have classes that last 45 minutes, or 90 minutes, depending on context.

• Binn's Magical History (single class) - 1.5 hours OR 45 mins.

  "Today, they suffered an hour and a half's droning on the subject of giant wars." (OotP; year 5)

  "Today they suffered through three quarters of an hour’s droning on the subject of giant wars" (OotP, a different edition! You can find BOTH versions of this sentence in different book versions!)

  The latter seems correct as details confirm it:

  Harry heard just enough within the first ten minutes to appreciate dimly that in another teacher’s hands this subject might have been mildly interesting, but then his brain disengaged, and he spent the remaining thirty-five minutes playing hangman on a corner of his parchment with Ron, while Hermione shot them filthy looks out of the corner of her eye.

• Double Potions with Snape - 1.5 hours (so 45 mins per single)

  "you have an hour and a half...start".(OotP; year 5, "Professor Umbridge")

  Note that this was entire class for the potion, according to next text:

  “A light silver vapor should now be rising from your potion,” called Snape, with ten minutes left to go.

• Double potions with Slughorn: 1.5 hours too

  “Well, now, this looks absolutely wonderful,” said Slughorn an hour and a half later, clapping his hands together as he stared down into the sunshine yellow contents of Harry’s cauldron (HBP)

  (it says they had double potions in Chapter 9 of HBP: "They had only just finished when the bell rang for the afternoon’s double Potions")

• Double Charms with Flitwick: 1.25 hours + padding (so likely 1.5 hours)

  Double Charms was succeeded by double Transfiguration. Professor Flitwick and Professor McGonagall both spent the first fifteen minutes of their lessons lecturing the class on the importance of O.W.L.s.... They then spent more than an hour reviewing Summoning Charms, ... and he rounded off the lesson by setting them their largest amount of Charms homework ever. (OotP)

  Plus your own quote from GoF, Year 3

  Being shut in a dungeon for an hour and a half with Snape and the Slytherins

• DOuble herbology is 1.5 hours

  Tired and smelling strongly of dragon dung, Professor Sprout’s preferred brand of fertilizer, the Gryffindors trooped back up to the castle an hour and a half later, none of them talking very much; it had been another long day (OotP, "Detention with Dolores")

  Their robes billowed and swirled around them as they splashed across the flooded vegetable patch to double Herbology (OotP, "Dumbledore's Army")

There was also "hour" long lessons as you noted; but that could simply be 45 min long class that ran extra.

• Moody's DADA

• Hagrid's flubberworm CoMC in 3rd eyar

  “Why would anyone bother looking after them?” said Ron, after yet another hour of poking shredded lettuce down the flobberworms’ slimy throats (PoA, Chapter 8)

• Then that same year there was 2-hour-long Care of Magical Creatures class too!

  Classes started again the next day. The last thing anyone felt like doing was spending two hours on the grounds on a raw January morning, but Hagrid had provided a bonfire full of salamanders for their enjoyment, ... (PoA)

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However, the real number is most likely 45 minutes:

1. As seen above, double portions with Snape AND Slughorn are 45*2, in 3 different places, as are double Charms and double Herbology; and single HoM is 45 mins.

2. I have previously read references that in Scotland class length is 45 mins

3. Moody's extra-long session with Harry was, as you said, 1 hour, meaning normal class was less than that.

4. If a single class is 1.5 hours, we have a problem of not having enough time in a day without Time-Turner:

   Day's lessons in OotP are "History of Magic, double Potions, Divination and double Defence Against the Dark Arts". (OotP).

   Also, "Detention with Dolores" chapter in OOTP has double Charms, double Transfiguration, Care of Magical Creatures and Herbology on same day, same configuration of 2 doubles and 2 singles.

   If single class is 1.5 hours and double 3 except Potions, we'd have 7.5 to 9 pure study hours/day. Add in extra time to go between classrooms at Hogwarts (45 min) and lunch (60 min); and you run into 9+ to 10.5 hour long school day. 9am-6:30/7pm. That's extremely unlikely even in a Another Brick In The Wall Hogwarts environment. Especially since Harry's detention with Dolores was at 5pm.

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Bonus round: Lunch is 1 hour:

Harry spent the rest of the lunch hour sitting alone underneath the trapdoor at the top of North Tower (OotP)

Harry and Ron spent their lunch hour in the library looking up the uses of moonstones in potion-making (OotP)

He had to give up his lunch hour to complete the picture of the bowtruckle (OotP)

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Bonus Round 2: Normal exams are 1 hour:

Their very last exam was History of Magic. One hour of answering questions about batty old wizards who’d invented self-stirring cauldrons and they’d be free, free for a whole wonderful week until their exam results came out. (PS)