if you can't judge a book by it's cover, when can you judge it?
I used to finish every book, no matter what, as a point of pride and for that secret gold star from the universe . . . and then I got older and realized life is short. I believe my 3rd eyeroll/WTF is quitting time, or when my mind starts asking "do I care about any of these characters" and the answer is NOPE.
After finishing Cheryl Strayed's Wild, I raged against the universe. And vowed never again. Just the inklings of an eye roll and a book is toast. But someone spent months or years writing that book, and so I feel guilty every time.
I loved this post! I generally finish books, but I'm fussy about which ones I start. I'm reading one now that almost had me using my Kindle as a Frisbee in the first chapter. But suddenly it got better and I'm staying with it. It fits within my 50 page rule. If it's still bad after that point, or just hasn't grabbed me at all, I have permission (from myself) to abandon it.