I don't have a hard and fast rule as to when to abandon a book. I'll stick for a while, but at a certain point I just know it's time to say, "Nope. You're not for me", and move on to the next one.
There's too many great books to get hung up on one that bums me out, and slows me down. By abandoning books I don't like, I'm making more time and room for the ones I will like.
That's why it's okay to bail on a book you're not enjoying. Just let that one go. But please, pick up another one. It's worth it when you find one you love.