Basically, sometimes one may find out too late that a book isn’t quite up to one’s taste. The book might not be gripping enough due to many factors. In such case, would you quit reading it? Does the amount of progress you put in make a difference? Or would you be nontheless determined to complete despite not feeling any meaningful connection?
For fiction it depends how far along in the book I am when I lose interest. If it’s still early on and I am not feeling any connection to the story or characters then I can drop it. For non-fiction I can drop it at any point I lose interest.