One reason I'm asking is from a writer angle. How much patience does the average reader have for getting into an unfamiliar book?

According to most readers and editors, about two chapters. If you haven't captured them, either with voice or plot or characterizations, they'll bail, unless, again, it's an author they're familiar with-- then, halfway seems to be the killing point.

ETA: Depends on genre, of course. Fantasy & sci-fi seem to have higher thresholds than say, romance, which is why I've had an impossible time selling a romance manuscript. I take my time setting up and weaving my stories and that seems to be anathema for the genre

Then there's that whole pesky, I don't follow the "rules" of romance worth a damn that tends to be the nail on the coffin.

I almost gave up on a book I just finished. It didn't start getting interesting until page 200, and it only had 300 pages. I didn't give up only because Patricia Briggs blurbed it...which makes me less likely to pick up a book in the future if I would have been interested just because she blurbed it.

That's awfully convoluted.

I was recently reading a thriller that I almost gave up on, but it was a fast read and I soldiered on. However, when I was on page 100, I yelled, "For the love of god, kill somebody." If it's a thriller, you have to have actual danger early on, not 100 pages of someone thinking a new doctor is creepy.

This book also, memorably, described a period of silence as "turgid" and a sky as "tumescent." There was some unresolved sexual tension in the book, which expressed itself in random adjectives.

Speaking as a reader (as opposed to someone who's actually written something), I usually try to get to the end, even if I skim a lot. But there are some books ... sigh ... either the plot has me thinking been there/done that without any redeeming new element, or I really dislike the main characters (and there aren't any interesting secondary characters), or it's badly written, or I find myself compiling lists of anachronisms or of misused words, or something about it makes me hate it I'll give up.

I never give up on a book. It's a failing, actually, because I waste a lot of time reading really really subpar books. But I read fast, so it's okay. Closest I ever came was Cryptonomicon or whatever the hell that hot mess was. I threw that book (a shocking act from must-not-damage-the-books me), but I finished it. And that was a damn lot of pages.

I read fairly quickly, too. But I've come to accept that some books just aren't worth the time. I recently read a rather odd book - "The Cowboy and the Vampire" - that I'm still iffy on, but it held my interest enough to finish it. But a little while ago I was reading a romantic suspense book and found that I disliked both the hero and heroine enough, and could see exactly where the story was going, that I gave up on it.

"This is not a novel to be tossed aside lightly. It should be thrown with great force." - Dorothy Parker

I loved Cryptonomicon, but I had a high fever while I was reading it, and keep trying to crack codes in my sleep.

I used to slog through them all, but I've given up on a few in the last few years. One was a mystery that was written almost entirely in similes.

Well, I came to the realization today that I just need to give up on the idea of ever selling a romance manuscript-- at least in today's climate-- after reading a review for an Angel/Mermaid story that apparently involved the gentle brushing aside of her scales in order to get to the glittery hoo ha of happiness, and seeing all the enthusiastic oohs and ahhs from the readers.

And when I submit what I think of as a good, solid contemporary romance to an editor who wants good, solid contemporary romance and receive a rejection because it's "too real and ordinary."

Well, damn, of course, it's going to be too real and ordinary compared with scales and angels and glittery hoo-has.

that apparently involved the gentle brushing aside of her scales in order to get to the glittery hoo ha of happiness

Ew! Ew!

Ew! was definitely my response.