How much is enough?

So I picked up a bunch of novels about a week ago at the library. I was a little nervous because I usually don't like novels. Well, to be more accurate, I like them but I could never actually admit to reading them because I wouldn't recommend them to anybody else.

Example: One Thousand Acres by Jane Smiley. I LOVED this book. It was such a thought provoking and amazingly crafted novel. Having studied King Lear a couple times in college and attending some different performances, reading this modernization of Shakespeare's play was mind blowing. Every detail was perfect. It was obvious why it had won the Pulitzer. It was also heartbreakingly sad and put me in a funk for about a week after finishing it. And I had even skimmed the disturbing parts.

So, one of the novels I picked up last week was like A Thousand Acres. It's called Ahab's Wife and it is an amazing novel. Over six hundred pages long, it was inspired by a paragraph in Moby Dick. Since it's neither a sequel nor a prequel, I guess you could call it a companion novel. The book is beautifully crafted, rich in sensory details and the title character, Una, is not one I'm likely to ever forget. However, after reading about one abusive father, two suicides, three dead babies, the gory process of hunting, slaughtering, and rendering whales, and a stint with cannibalism, I had to put the book down.

Which brings me to my question: How much is enough? When reading, all the action happens in your head and is limited to what you know and percieve. The text can enhance what you know and percieve but it can't give you what you don't already have. Take color for example. Imagine if you had been born seeing only black and white. No amount of description could explain red. All you would understand is the shade of gray that represents red in your mind. Whale rendering and cannibalism pretty much work like that for most readers. But abusive parents, dead babies, and suicide don't. And, if the writer is worth the paper their novel is printed on, they will make the things you can't relate to (like cannibalism) relatable (which the author does by comparing it to an earlier portion of the book where Una eats a goat she has raised since it's birth). Cannibalism is still a shade a of gray for me, but that doesn't mean I want to read about it.

So again, how much is enough? How much description is enough? As the author, how do you know how much sadness is enough? As a reader how do you know how much horror you can take?

And really, is there any way to know how much is enough before you've had too much?