It depends. I’ve given up after the first page. I’ve given up halfway through the book.
I love to read. I live to read. And if I’m finding that I’m not looking forward to reading the book, it’s time to give it a miss and move on.
Heh. I once gave up about 100 pages from the end of a big fat fantasy trilogy. It was that Prince of Nothing (or something of Nothing) thing from (somebody) Bakker. I don’t know why it took 1200+ pages to get sick of the self-indulgence, but it did.
Bad writing (poor grammar, misuse of words, etc) will turn me off within the first few pages. I haven’t got through much of the prologue of the Da Vinci Code or past the first page of the The Bridges of Madison County
Failure to do the research (factual errors in basic science/chemistry/whatever that your average high-schooler would catch) will generally cause me to figuratively hurl a book across the room. If the flawed fact is a plot necessity I’m an iota more forgiving than if the writer was just too lazy to use unflawed facts to accomplish the same end.
If I read until I fall asleep and have no desire to find out what happens next the following day, then I consider the book a lost cause. This usually happens within the first three chapters or 50 pages.
If I find the main character too morally repugnant (and I have read the First and Second Chronicles of Thomas Covenant) I’ll toss the book too.
All that said I must confess that I do read most of Clive Cussler’s output, mostly because I can turn most of my brain off and enjoy the ride. So if the story is good enough and I have a history with an author’s books I’ll forgive clunky dialogue, the occasional torturous paragraph, and even the basic science errors. But an author new to me doesn’t have nearly the same latitude.
-DF
Ivylad likes fantasy novels, and I tried to read one of his trilogies. If the book is sitting by my bed with a bookmark in it, and it doesn’t call to me to open it, the bookmark is pulled out, the book is shelved, and it’s on to other things.
The other thing I do that Ivylad doesn’t is reread books. I cannot tell you the number of times I have read Gone With the Wind, or Mists of Avalon, or the* Outlander series*, or The Thorn Birds. I love rereading good books.
Depends on the type of book (fiction, non-fiction, science, etc.) but usually the yellow alert crapometer will light up within the first few minutes, red “go ahead and toss it down” confirmation buzzer will usually follow within the next few pages.
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I am the exact same way. I get a ton of books from the library and finish about a quarter of the ones I start. And I’ve also got video games to play, and a weird attention span. If something doesn’t interest me I will wander off. But when something is good, it’s sublime. It’s worth it to start 10 books I’m not interested in just to read one amazing book.
The fastest I ever put a book down in disgust was two sentences.
Book was A Rustle In The Grass, about good and evil and anthropomorphic ants. I think. Definately about the ants. Now, you’d think that someone writing a book about ants would do some basic research first, right? Wrong. Second sentence used a male pronoun for a worker ant, which made me realize that the guy had no idea what he was talking about, which made me stop reading before I had a brain explosion.
Mine was one sentence: Jpod by Douglas Coupland.
Carlos Ruiz Zafon’s “The Angel’s Game”. The funny thing was that the book kept alluding to something better underneath. It was playing peek-a-boo with an interesting story. I saw a flash and it kept me going to the very end where I chucked it into the trash. I didn’t want anyone else to feel that same disappointment that I did.
And grayhairedmomma nailed it on the head about “Twilight”. I couldn’t get through the book and the only reason I watched the movie is because I got a copy with Rifftrax. Our daughter is insane about the series. Y’know how some people say “at least they’re reading”? Yeah, f— that when it comes to Twilight.
It’s pretty rare for me to find myself reading a bad book in the first place. I know enough about books and my own tastes to be able to pick out good ones most of the time. I can see how this would be more difficult for someone who didn’t read a lot and didn’t have favorite authors or genres to fall back on, but for a bookworm I think it’s usually pretty easy to avoid books that you’re going to hate.
If I am reading a bad book it’s often because I have some reason to be reading this particular book (it was a gift, I want to compare it to the movie version, etc.), so I’ll usually plow through even after I’ve realized it’s bad. It doesn’t take me that long to read a book. There have also been a few times where I finished a bad book because I had nothing else to read, because it was bad in some way that I found interesting (if only in a “what not to do” kind of way), or because I wanted to make fun of it later. 
If the actual writing style is bad this will be obvious very quickly, often within the first few pages. A bad story can take a little longer to recognize, especially since some books are slow to get going. There are also some books that start strong then lose steam, but if they’re really bad this will usually become apparent within the first couple of chapters – say 20-50 pages. That’s just an estimate, I have been taken in by books that seemed okay longer than that. If a book seems bad I will sometimes skip ahead to see if it seems like it’s going to get any better later on. This has occasionally convinced me to press on, but has more often confirmed that the book really is bad. I may abandon the book then or skip to the end just so I’ll know how things worked out.
The last book I started but didn’t finish was, surprise surprise, Twilight. I could tell it was bad from page one, but I might have finished it anyway under different circumstances. I was visiting a friend who’d borrowed it from her teenage stepsister, and my friend, her husband, and I took turns reading it out loud every evening I was there. This took a lot longer than reading it silently (especially since we had to keep stopping to laugh – my friend actually fell off the couch at one point) so we only made it about 1/3 of the way in by the end of my visit.
Before that I don’t remember the last book I started but didn’t finish. Whatever it was I probably just read a chapter or two before casting it aside and forgetting all about it. I remember doing the skipping-around-to-see-if-the-whole-book-really-was-that-bad thing with Anne Rice’s Memnoch the Devil, but that was more than ten years ago.
I must know: what was the sentence?
Honestly, I can usually tell just from the book jacket if a book is going to be crap, but barring that I can tell within a page or less whether the writing is any good. I won’t necessarily toss the book if the writing is bad because sometimes bad writers can still be good storytellers (and vice versa). At most, it might take me 10 pages to really know what I’m reading, but I generally don’t have to get past the blurb.