How long should a TV series last?

If you look at great UK tv shows like "The Office," or "Fawlty Towers," or "The Prisoner" none of these shows exceed twenty episodes -- and I think they're better off for it. Their should be a limit I feel to how long a show should last -- maybe 50 episodes tops. Otherwise you get shows like "The Simpsons" and "Gunsmoke" that overstay their welcome and become so stale and parodies of themselves. I tried watching "The Andy Griffith" on Netflix recently, and while I enjoyed it, after about 50 episodes (the shows is well over 250 total) I felt I had had my fill of Mayberry. The same goes for "Friends": good show but 50 or 60 episodes of these characters is all I'll ever need ... all, I feel, anyone would ever need.