How Dark is Too Dark?

I believe (tho am not sure) that urban fantasy is considered very dark. Some paranormals, too. This is neither urban fantasy nor paranormal, but an historical. So my question is: How dark is too dark?

Where's the line? What's considred too much?

On the other hand, what do you want to see? How deeply do you want the angst to run? How horrible do you want the world to be?