I have a 3 episode rule! If I still really don't like it after 3 episodes, then I generally give up.
However, I know there have been times that I stuck with a drama I didn't necessarily love because I had hope that it would turn out better. Or something in the drama happens that piques my interest, and even though 80% of it is boring me, and I'm on my phone, I'm waiting to see if that other 20% will wow me. And that's how I sometimes end up finishing a drama and then going, "Why did I watch this?"
But if you look at my drama reviews, you won't ever see anything rated below a 6. That's because I don't bother to finish dramas that would be a lower rating to me! ^.^