Now I'm trying to forgive my adolescent self for all those Piers Anthony and John Norman novels.
Anyway, here, according to Goodreads, are the longest novels I've read. (I excluded textbooks, non-fiction, etc., from the list.) Note that these are only approximations of length, since page counts vary between different editions. Also, an ebook will have a smaller page count than the same book in hardcover or paperback, and children's books (such as Harry Potter) have fewer words per page and therefore aren't really as long as an adult novel with the same page count. Audiobooks, of course, don't have page counts at all.
- Cryptonomicon, by Neal Stephenson (1168 pages)
- The Stand, by Stephen King (1153 pages)
- It, by Stephen King (1104 pages)
- The Way of Kings, by Brandon Sanderson (1007 pages)
- Harry Potter and the Order of the Phoenix, by J.K. Rowling (870 pages)
- The Illuminatus! Trilogy, by Robert Shea and Robert Anton Wilson (805 pages)
- Shogun, by James Clavell (802 pages)
- Harry Potter and the Deathly Hallows, by J.K. Rowling (759 pages)
- Bag of Bones, by Stephen King (752 pages)
- The Tommyknockers, by Stephen King (752 pages)
What are your longest? I doubt anyone can top this guy. Though if you want to give it a shot, you could try Marienbad My Love or The Story of the Vivian Girls.
ETA: I just remembered that I read The Mists of Avalon (876 pages) in high school. That should be in the #5 position, knocking The Tommyknockers off the list.