When to Quit a Book

Began reading a book called "Centaurs  (Parallel Worlds)".  The dialog was so stiff and preachy.  I stopped reading it before my usual stop of 1/4 of the book or 50 pages.  At that point, if I don't care how the book ends, it is not worth my time.  What other criteria have you used?  Well, I must confess, if I have paid for a book, I feel compelled to get my money's worth and read all of it.  Used to think if an author went to the trouble of writing the book, I should finish it. Too many good ones out there I want to read.