I had to read Anna Karenina by Tolstoy in the tenth grade or something. It was 817 pages of long. I'm currently reading one on the six wives of Henry VIII, 765 pages. Depends on how tall the book is and the size of the print and such for how much you're actually reading though. Word count would be more accurate, but a hell of a lot more difficult.
Edit: I read my biology textbook pretty much front to back last semester because I'm a huge nerd. It was 821 pages.
Cryptonomicon had the most pages of anything I've read in the past few years (1152). It took me a month to get through. I don't know if it's the longest ever, though. Longest recent non-fiction would probably be my Behavioral Neuroscience textbook.