Long-runners work to the extent that they are soap operatic and viewers can tune in for a single arc and feel happy. Look at manga and its various adaptations t

Long-runners work to the extent that they are soap operatic and viewers can tune in for a single arc and feel happy. Look at manga and its various adaptations to screen for examples of how this plays out long term: While many successful books will get a "one cour" (12/13 episode season) TV anime treatment, enough to be an advertisement and depict some major scenes in abridged fashion, the most popular ones get the 100+ episode exhaustive adaptation. The vast majority of those are shonen battle series of the Dragon Ball Z mold. In those, expectations are set for each arc being relatively contained and focused on the development of a few characters or enlargement of the overall cast. The plots can be complex but usually not so much that you are relying on knowing the whole thing, because it will helpfully flashback that information in nearly every episode. And like a wrestling lineup, if one character has exhausted themselves and their development, you just sideline them and add a new one. The individual plot arcs can say something on their own but they work through an overall supporting framework and formula. The stuff that's relying on being dense and literary, with an overall point being made, tends to need to get the job done within a few hours of runtime. 6-12 hours, like you say.