I rarely give up completely on books, but when I do, I try to read at least one third to a half of it to give it a fair chance. If after that I'm still not interested in the characters or what happens to them, then I'll give up. For a non-fiction, I might skim read a couple of chapters to see if it improves, or look ahead to see if there are any sections I'm particularly interested in reading before deciding to give up. Some books I know I want to read, but after having started them I find I'm not in the mood, so I'll put them aside and come back to them, but I rarely do that either, as I find it difficult to go back to a book once I've put it aside.