Depends how close the show is to the original theme. I loved Charmed growing up but it stayed a little too long. Some shows that need to die... LOST, thank god its the last season, the universe better not reset into an endless loop or something like that. As for anime I like to stay with 12 or 24-26 episodes (1 season) a show.
I think its better to want more, than to lose interest. Of course marketing departs everywhere would beg to differ.