Done well, a long novel can be immensely satisfying and offer a chance to explore a character in greater depth than is possible in three hundred pages. Part of the emotional weight of War and Peace, for example, comes from the exquisitely long time over which the story unfolds. However, a long novel does take stamina and perseverance. At The Millions, Mark OConnell has written a good piece about the challenge of long novels:
"I used to take a wan pleasure in telling friends who had returned from a tour of duty with War and Peace or The Man Without Qualities with that I’ve-seen-some-things look in their eyes—the thousand-page stare—that they had been wasting their time. In the months it had taken them to plough through one book by some logorrheic modernist or world-encircling Russian, I had read a good eight to ten volumes of svelter dimensions. While they were bench-pressing, say, Infinite Jest for four months solid, I had squared away most of the major Nouveau Romanciers, a fistful of Thomas Bernhards, every goddamned novel Albert Camus ever wrote, and still had time to read some stuff I actually enjoyed."
Eventually OConnell began reading longer books and makes this interesting observation:
"When you read the kind of novel that promises to increase the strength of your upper-body as much as the height of your brow—a Ulysses or a Brothers Karamazov or a Gravity’s Rainbow—there’s an awe about the scale of the work which, rightly, informs your response to it but which, more problematically, is often difficult to separate from an awe at the fact of your own surmounting of it."
That's true to an extent but it also doesn't really matter. The scale is an intrinsic part of the work and responding to that is as valid as responding to the characterisation or the prose or the plot.
OConnell's characterisation of being in the midst of a long novel as a kind of Stockholm Syndrome is not entirely convincing. It assumes, for example, that stretches of these books are intrinsically torturous, which I'm not convinced is true. However, his argument is witty and worth reading.
I've read a lot of long novels and people occasionally ask me how I do it, as if I've climbed a mountain or something, rather than simply having turned a lot of pages. Leaving aside the difficulty of the work - there are 200-page novels that are difficult to read too - it really is just a matter of starting at the beginning and reading one page at a time until you come to the end.