I just read this essay that says there's a cave that takes over 7 hours to "thoroughly" explore. Which cave is the author referring to?
There's at least one cave that takes upwards of seven hours to thoroughly explore.
He is probably talking about Blackreach.
As taken from here:
You gain access to Blackreach through the ancient Dwemer ruins of Alftand. The quest Elder Knowledge will take you there, and it’s the most bizarre, fascinating place in Skyrim. Here you’ll find the ruins of an immense Dwemer city, weird jellyfish-like glowing plants, and one of the game’s rarest items, Crimson Nirnroot. Picking one of these will start a quest, A Return To Your Roots, but watch out for the Falmer, Spheres, Wisps and Chaurus that dwell there.