When has a joke gone too far?
Assuming you believe it can.
Everyone has a different definition of 'too far'. Some people are more tolerant and less sensitive than others while some people do not take jokes very lightly. It all depends on the person, in my opinion. Personally, I am quite comfortable with jokes of any kind except for jokes about serious illnesses. I am also not a fan of ill-intentionally racist jokes.
When the content of the joke is about something tragic that has recently happened, then it should be off limits. Such as a 9/11 joke before the bodies are even cleared from the wreckage.
One can joke about the government's handling of it, the engineer of the building, but leave the victims out of it. They should be forever off limits.