I have that wonderfully helpful answer... it depends.
Sometimes I know it's a mood thing (there are a few books from when I first gave up smoking that I really must give a second chance). I think the very best books, like the very best movies, make their own mood ... but lesser works, works by mere mortals, have to rely on finding their readers/viewers in a receptive mood. If you think it's mood, put it down for now (before you get a really bad feeling for it) and come back another time. But I would not do that twice, if it can't get me on the second attempt it's got serious problems.
The other thing it depends on is expectation. A thriller, a mystery and even most fantasy, must get me in pretty quickly. If not with excitement then at least with some interest or intrigue. In the case of other books, particularly historical fiction, I find that I often have to give them more time. With historical fiction it sometimes takes a while for my brain to switch modes, but once that happens I find that suddenly the prose begins to read more smoothly and the whole tale begins to take on life and feeling.