This is a somewhat recurring theme, so I'm following Anton's advice and starting a post.
At what point do events become so long and challenging that they become unhealthy (mentally, emotionally, and/or physically)?
I'll leave it at that and post my own anecdotes to follow. I don't have a firm opinion on the matter, and I know it depends on everyones personal situation. I would say a deca-ironman is certainly not a healthy endeavor. I would say a marathon might even push it for some people, but is still within the boundaries of healthy for most. I think an ironman is over the boundary for most , but still within the boundary for a few.