It does depend on the person. It just so happens Zack is flapping over it a bit. He's used to pain. He carries it with him, having absorbed it when he works his apocalypse mojo. To think it's something else that he doesn't know about though, is a little disturbing.

"But I am what I really am," comes out of his mouth before he thinks. Zack assesses what he's just said and decides to go with it. "What else would I be?"

Putting down the burger, Zack rubs his jaw with his fingers, as if that will tell him that he's actually what he is, or whatever. Then realising how dumb that is, he asks instead, "How much time? for it to come apart?"