What readily comes to mind is Atlas Shrugged by Ayn Rand (who I like a lot btw), it took me, I think, 23 days. Which is a long time for me. I know it took me ages to read Clive Barker's The Great and Secret Show too, but that was about 18 years ago and I was hit by a car . (I was on foot!) It was good though and I enjoyed it. Even The Stand didn't take me longer than 10 days.