I think that most Anericans would take this to be asking when someone had last emptied their bladder. Even then it would be more natural as "How long is it since you last went to the bathroom?"2) But what if we use this form? Can we say it's a humorous way of asking the question above and shows more surprise? Actually I'm looking for a humorous way to say that.
"How long haven't you been to the bathroom?"
Student or Learner