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?
About 10-20% for me. If there’s nothing that I can latch onto by then, the rest is almost certainly going to be a waste of time.
I used to always consistently read audiobooks at least till the end, but now I know when to not waste my time.