I agree that length alone is not a good way to determine how successful a relationship is. I do believe it can be a factor depending on the goals and preferences of the people involved, but that important shouldn't be applied by people external to the relationship. In other words, some people might want long-term relationships and therefore a short-term relationship might bring them less satisfaction. However it's for the people having the relationship to decide.
At any rate, I'll definitely support breaking off a relationship before it goes sour over keeping the relationship despite it being the people involved miserable. I think people tend to put the sake of a relationship over the sake of the people who are inside that relationship, which in my opinion is utter madness. A relationship is meant to serve the people, not the other way around.