I have learned that a show should never go more than 5 or 6 seasons. They always become awful after that. They should also have a satisying ending. Breaking bad, and the new Battle Star Gallactica ended with a real ending with the correct amount of seasons before they got stale. How I met your mother on the other hand, should have ended at least 3 seasons before they did, but it at least had an ending.
Curious observation. I've noted that 6 years is roughly the time it takes for even some of the best members of Congress to go stale (with few exceptions).