In recent memory, it would be The Fry Chronicles by Stephen Fry. 397 pages (though I know pages are relative to format). When I was young (high school), I had The Gulag Archipelago by Alexander Solzhenitsyn (yeah, I've always been "unusual", lol), which is also very long, but I don't remember if I made it all the way through.

Edited to add -- woops, looks like Dead Souls by Nicolai Gogol weighed in at 402 pages, edging out the Fry book. It was a slog making through this book put out in 1842, but I was determined.

Do you like Halloween?