For me, all books start as five stars. They can lose stars for various egregious flaws (minor flaws get a pass unless there are so many it becomes an egregious amount). If a book drops below three stars in my mind, I won't finish it.
Today I didn't finish one. The 1st star dropped because the main character was pretty much infallible. The 2nd because the plot wasn't advancing beyond action scenes where Ms. Infallible remained so. 3rd was a gratuitous torture scene and I was done.
@ianthealy Good way to look at it. I just struggled through the 10th book in a series because the first half was repeatedly repeating what had happened in earlier books. Thankfully, it finally picked up in the second half. I'm invested and don't want to quit.