The 30% rule. If it's still hasn't grabbed you by the 30% mark you can walk away without feeling guilty. You've been fair to the author and the book and even if it's good it's probably just not your thing.
I quit two books just yesterday because they either pissed me off, or bored me and irritated me. I've got so little patience. If there's not something good pulling you through - quit. Don't force it. That's my new rule. LOL
30% is my rule of thumb mark too. Sometimes I make it there, others I don't, but I never feel guilty for giving a 1/3 of the book a chance and not wanting to go on.
More and more I find myself going back to re-reads because I just can't stand anything new I'm picking up (unless it's by an author I already love). I worry (a little) that I'm going to become unadventerous and *only* read authors I know and love pretty soon.
30% is my rule of thumb mark too. Sometimes I make it there, others I don't, but I never feel guilty for giving a 1/3 of the book a chance and not wanting to go on.