As a child it was a rule at our house that you had to finish a book no matter what. This stood me in good stead when I had to read books for English Lit that were not of my choice. It was a rule I followed unquestioningly until I was in my 20s.
I can't remember what triggered this but I decided that reading was for pleasure and entertainment and why should I make myself finish a book I didn't enjoy?
I usually allow 2 or 3 chapters and if I'm not eager to pick the book up again I move on to the next one.