A personal rule has been, for years and years, finish the book. If it was interesting enough to start, give the writer a chance and finish it. This applied specificially to novels, of coruse.
Lately I've started a few novels and decided not to finish. One became too slow, too repetitious; another seemed to me the work of an author who incompletely imagined the character's situation. I wasn't sure where her idea of story came from, wasn't sure quite what the time frame was but some scenes rang so untrue it seemed a young writer was copying situations of which she had absolutely no understanding. Those are the two I've most recenty stopped reading.
Last night I turned to a book by Paul Auster, Sunset Park. He is a writer I know I can trust. His characters will be true, they may surprise me in the best imaginative way. In the case of the two I stopped reading, they were by non-American authors; I had chosen their books from a library book sale on the basis of the cover information, both were first books. Sometimes these are wonderful. But I've given myself permission to move on when I find myself resisting the book.
The cliche is true: there are so many books and I have so little time ... how much I don't know, of course, but being mid-70s, it's an ever shortening time. The world is full of wonderful books so I need not put my time into ones that I cannot feel excited about. Just now I am finishing Oliver Sack's Musicaphilia. Although he is repetitious, I am learning a great deal as I always do from his books, and his many case studies fascinate me. Sacks and Auster are both writers, nonfiction and fiction, that I've enjoyed before and continue to enjoy.
The mid-70s are a surprise! Part of me remains in the 50s -- age, I mean, not decade of 20th century. It's a joy ride, new experiences land in my lap and I've become a better quilter, poet, writer than I expected. It's a rich life for a person never rich financially. Hey, this is what the mid-70s are like!