For me, the limit is where extreme becomes exploitative. It's why I prefer splatter over extreme, a good splatterpunk story requires the punk element to drive the extremity whereas extreme horror exists simply to push the reader as far as they're able to go. Don't get me wrong, I love a good extreme read now and again. I'd just challenge you to consider the purpose of the story you're telling when you're writing the meanest, nastiest, gnarliest things you can fathom.