My preference would be that a series end before it becomes a parody of itself. To me the signs that a show has gone on too long are:
[ol]
[li]All of the major cast has been replaced by new or secondary characters[/li][li]It’s a drama, but they do a “funny” episode. More than once.[/li][li]They introduce a Cousin Oliver (from The Brady Bunch for those of you too young to remember) character who really is more annoying than useful to the plot.[/li][li]The show spins off (successfully or not) more than one other series[/li][li]The show tries to sneak in recycled episodes from earlier seasons.[/li][li]More than one Christmas special/flashback episode during the run of the series[/li][/ol]
Shows that ended before they grew stale: The Rockford Files,Wild Wild West,Fringe (five seasons were enough),In Plain Sight
Shows that lasted waaayyy too long: Happy Days, Friends, ER, Law & Order:SVU (still going, unfortunately),JAG, NCIS,Cold Case, Criminals Minds,etc