This question is asked a lot.  Today, it was asked by Cynthia Crossen in the WSJ, who asked it out loud after a reader asked her the same thing. Crossen doesn’t have a definitive answer for when one should give up on a book, but admits that she does, and also admits this doesn’t make her feel great.  

I also don’t always finish books, and this is becoming more and more common for me.  And I don’t feel badly about it anymore.  I’m busy.  I have a short attention span and there’s so much great writing in the world that I don’t want to waste my time bored.  Yes, you should listen to your friends when they tell you you’re wrong about a certain book and maybe give it another shot, but when a book isn’t vetted, I think it’s fine to put it aside, or toss it, until someone real-that is, not the reviewers on the front or back covers-says you’ve made a mistake.