There are a number of sites that say Dumbledore and Grindelwald's duel lasted 3 hours. For instance, The Humor Nation article “10 Harry Potter Most Powerful Wizards & Witches”:
Grindelwald eventually got his hands on the Elder Wand which he then lost when Dumbledore defeated him in a duel that lasted three hours long.
I reread all of Deathly Hallows but there was never any mention of the duel between Grindelwald and Dumbledore lasting 3 hours.
When Elphias Doge mentions the duel, he simply states that the onlookers wrote of the terror and the awe of watching the two extraordinary wizards duel, but not that the duel lasted 3 hours.
And when Dumbledore and Harry talk in King's Cross, Dumbledore simply says:
“I think he knew it, I think he knew what frightened me. I delayed meeting him until finally, it would have been too shameful to resist any longer. People were dying and he seemed unstoppable, and I had to do what I could.
“Well, you know what happened next. I won the duel. I won the wand.”
Again, there was no mention of the duel lasting three hours, just that Dumbledore bested Grindelwald and took the Elder Wand and became its master. Where does it say Dumbledore and Grindelwald's duel lasted 3 hours?