If the book hasn't been recommended to me or if I haven't heard about it, I read the back cover and first sentence. If that doesn't catch my attention, I usually move on to the next book. But books that have built themselves a reputation... I give those a few paragraphs.
1 chapter minimum if I already commited to it (checked it out, borrowed or bought)
Of course the description had to catch my attention or I would never have started it.
The soonest I've ever put down a book, which is hard to remember as there's been so few, was after the second chapter I think...or between the first and second chapter. Movies, on the other hand, I've walked out or turned them off before the first ten minutes were done.
Never after the first sentence, but often after the first few pages. If a writer can't hook me in the opening ten pages, life's too short to simply soldier on.
I've never read Catcher in the Rye but I've seen the first sentence because it's always offered up as the perfect example of what an opening sentence should be. It made me want to walk away immediately.
I'm a glutton for punishment when it comes to reading. I hate not finishing something, and this can be especially problematic when it comes to a series. However, if the book is bad enough that I can tell there's going to be nothing promising whatsoever, then I'll put it away after a few chapters.