If a book doesn't appeal to me I quit. I give it a fair chance, 1/3 to 1/2 of the book. I must admit I rarely do it. Then again, I never ever buy on impulse. I always do a good research before I acquire a book.
I have checked my Goodreads bookshelves. There are approximately 300 books and only 6 are marked as unfinished. Although, to be honest, there is a dozen of books I was not supposed to read. It was a waste of time
I deeply regret of reading them, when I could enjoy their betters.