I think that a lot of writers have trouble with the needs of a long series, especially pacing and maintaining constant forward action. I'll take some examples from manga. Bleach, for instance, has gotten its feet tangled in all its characters, many of whom are there just for giggles. One Piece has done a good job, by contrast. it also has a large cast, but the enter and leave the series as appropriate. The addition of the latest member of the straw-hat crew was foreshadowed all the way back in like, Volume 1 or something. Since we know where Luffy's going and why, and since there's a reasonable amount of hinting and foreshadowing that goes on well in advance of the plot unfolding, things largely seem to be going somewhere and things don't feel made up as it goes. Monster has similar things going for it.
At the end of the day, a story should be told in as long as it takes to tell that story. So I guess if I had to give a rule I would say, "Anything goes, except for filler."