Unless I absolutely can not under any circumstances get past the first couple of pages (girl with a dragon tattoo and agatha christie books are the notable ones) I stick with it for about 100 pages, or 1/3 of the way through. ONE Notable exception- Sunshine, by Robin McKinley. I tried, tried to finish it. I really did, normally I LIKE her books. I got halfway through and had to stop. It was too inane and prattling for me. I hadn't even heard of Twilight yet, but looking back upon it, that's exactly what it is like, Twilight for ages 18+ (and now there is a YA version of the book, blech).
Cheerfully stolen from Robin McKinley's novel Sunshine, which I am re-reading for the *n*th time as we speak.