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.