Mark Billingham said he gives a new book 20 pages. If the story hasn't set up its plot, given him characters worth following and prose he cares about then he tosses it aside. 
Now Mark is a crime writer who knows his craft. The successful crime novelist's books regularly feature in best seller lists and his Thorne novels were turned into a television series. However, he said his wife disagrees with him. Speaking at this year's Cheltenham Literature Festival, he said that she soldiers on through an uninspiring novel as if it were a 'war on attrition.' Some of my friends say it is a 'duty' to continue on. They have started, therefore they will finish.
I have to say that I am with Mark Billingham. The job of a novelist is to draw the reader in, give us a hook, a promise of engagement - we want to follow your story. If the writer fails in this, then many of us have other books to turn to.
I have given up on many titles. I was once 450 pages into a 950 page novel when I decided I couldn't go on, I no longer cared. And it wasn't about its length. I have read War and Peace three times and thoroughly enjoyed it each time.

What about you? Do you soldier on till the bitter end like Mark Billingham's wife? Are you like Mark and give the text 20 pages? Or push through a little further until you no longer care and quit reading?