There are definitely long books I've read where I didn't feel like I got enough out of them for it to have been worth it, like I read (listened to) Gravity's Rainbow right after and a lot of the characters and plot lines just annoyed or disgusted me, but I'm sure that book means a lot to a lot of people. In the interview "Of course you end up becoming yourself," the author talks a lot about how in avant-garde literature you have to make the payoff good enough to reward the reader for doing the work, and that equation is going to be different for everybody.