How far along would be a good time to give up on a book?

I believe if you're not enjoying a book, it's perfectly fine to give up reading it. There are so many books out there, no need to waste time on books you can't finish or don't want to finish. So how far along would be a good time to give up on a book? 10%? 20%? Or maybe 40%? I had given up on books at 10%, 40% and even at 70% which some would say, you're almost there so why not finish it? My brain won't let me. There are scenes or things that I just can't get mind over it. But just because you give up on a book doesn't mean you can't go and pick up that book again later on. Here are the books I have given up reading so far this year. 

01 - A Death in Door County (Monster Hunter Mysteries #1) by Annelise Ryan > link
I was over 40% in when I decided I just couldn't continue with the endless descriptions and history of a lot places and things that don't seem to be related to the mystery. Mainly, I disliked the female main character - I think her name is Morgan. She seemed arrogant, egotistic, like a spoiled rich girl even if she claimed she wasn't. The would-be romance between her and the cop was subtle, boring and just unnecessary. There was one scene (not the exact wording as I don't have the book to check) where he said, "I'm attracted to you," and she replied, "I'm attracted to you too." As if he said, "I have a nose," and she replied, "Hey, I have a nose too." There is direct and there is this, whatever it is. There's nothing between these two characters except awkwardness. This cop is the so-called other main character/future love interest but he seemed very much like a side character because she basically bossed him around and he go out of his way to do things she wanted. I thought they would be partners in solving the murders but it was mostly her as she chose to do things on her own and even take evidence without telling him.
    [SPOILER: And they layered on the tragedy - she, the main character, had parents who were murdered by her fiancé, and he, the cop, lost his wife and child in an accident. And there's a third, semi-important character who lost his wife and child in childbirth. Three tragedies is a bit much. I skimmed to the end and was disappointed there's no monster, at least, not the one I expected. So this is a first book in a series but still, expectations. SPOILER ENDS]
    Anyway, it was just so boring. I think the bit about the main character's past was really the only interesting bit of this whole book. And the dog was a nice character. I like a dog who can swim and is the best buddy any pet owner can have.

02 - Miss Pym Disposes by Josephine Tey > link
I may have read about 30 or 40% before I decided I just don't care to continue. The writing is really good but I was just bored with how much mundane things can happen with Miss Pym and a college full of ladies. Miss Pym is maybe a little bit easily persuaded and even prideful because she was to stay at the college for a weekend but kept staying on because people kept praising her or making her feel like she's done a great thing even though she just wrote a book. There is a murder but I never got that far or even far enough to hint at a murder even though the book is 300 pages. I skimmed to the end and found out who was killed and who the murderer was and that's it.

03 - Stories of Your Life and Others by Ted Chiang > link
This short stories collection was boring and bland or perhaps I'm just too dumb to understand them. I think I had read at least three stories before I gave up.

04 - Where We Go When All We Were Is Gone by Sequoia Nagamatsu > link
The stories were too weird for me and confusing. You can't tell where one story begins or ends - they are like fragments or puzzles, they fit together but so loosely that they might not. I remember a character who can stretches his neck for miles and he was filming a couple having sex and I supposed he had the camera in his mouth because it was only his head that was there in the room and I just stopped at that story. It's just so weird. I really liked the cover though.

Have you any books you gave up on? How far along would be a good time to give up on a book?