If I'm reading a book, I normally give it a couple of chapters. If I don't like the style of writing or the story isn't engaging, it's very unlikely that it will get better later on.

If I'm listening to an audiobook, I give it a lot longer because I'm usually doing something else while listening and there's less effort required to progress.

A recent exception is The Power. I read that until 49% because it was okay. Somewhat interesting but I just didn't care about the characters and I hated the writing style. I thought I should like it more because it was a bestseller and turned into a TV show. I got around to reading reviews which confirmed that it wasn't just me who didn't get it and I'm so glad I stopped. I should've dropped it after two chapters.