Depends how long of a book it is. If you can't set up interesting characters or a premise within the first two or three chapters (unless they're oddly short chapters) I'm probably going to stop wasting my time on it. I don't have the time, hell I mostly listen to books now because there's more time for that than reading one. That said there have been times I've gotten over the hump of a slow introduction and it pays off.
Non fiction is a different beast. It really comes down to how well it's written and the information is presented.
50 pages is about right. Also if I don't see any improvement in the flow, I'll usually stop wasting my time.
Perdido Streets Station by Mieville was a good read, but the first 50pages were a fucking chore to get past.
I like to give everything a fair shot, because somethings are just slow builds, so if im like ~50 pages in and its still not going anywhere, thats when i call it quits.
Lol, I used to have a rule - if I'm not interested by the first 100 pages I'm out. But, I rarely followed it, today I only pick good books and if a bad one slips by I suffer.
Depends on the book. One time I noped out after working my way through a paragraph-long first sentence. Another time it took me halfway into the book before I finally realized the plot was only happening because every character was an idiot, reprehensible, or both. Since there seemed to be little chance they would all die in a fire, I was done.
I've been reading this Chuck Palahniuk book full of short stories, every other one makes me want to whip the book across the room because he's a retard.
If I'm not interested after character introductions and the inciting incident or hook of the story I'll stop reading.
This is honestly very generous as you have to be an extremely bad writer to fail at this.