I think Tim Parks--even as an aside-marks the border between a young reader and a mature one:
It seems obvious that any serious reader will have learned long ago how much time to give a book before choosing to shut it. It's only the young, still attached to that sense of achievement inculcated by anxious parents, who hang on doggedly when there is no enjoyment. "I'm a teenager," remarks one sad contributor to a book review website. "I read this whole book [it would be unfair to say which] from first page to last hoping it would be as good as the reviews said. It wasn't. I enjoy reading and finish nearly all the novels I start and it was my determination never to give up that made me finish this one, but I really wish I hadn't." One can only encourage a reader like this to learn not to attach self esteem to the mere finishing of a book, if only because the more bad books you finish, the fewer good ones you'll have time to start.
He goes on to talk about this problem as it relates to "good" books. But I'm not sure there is, in objective terms, such a thing. And even still, I think his rule still holds. It's often true that books improve as you delve in. But I don't think there's anything wrong with never making it through Ulysses.