Until there are no good stories left to be told, that's what I'm opting for. There are many possibilities which have not yet been made use of. New stories of course mustn't be mere repetitions of what has been told already, while on the other hand the repetition of certain elements cannot and need not be avoided.
I cannot give a number at which I would like for the series to come to a close. The number doesn't tell anything. If there are still good stories left to be told after episode 20, by all means tell those stories. If there are still good stories left to be told after episode 100 (unlikely), by all means tell the stories. If nothing good was left to be told after episode 15 (personally I'm convinced that there are more than 3 more good stories left to be told, but others may disagree on this) don't try to fudge a story.
