I am possibly over-influenced by my early experience of RPGs, where a campaign would tend just to drag on with no progress other than in power level – see previous threads in this category, particularly #3.
This advice is obviously mostly meant for writers of what one might call the indefinite series, the one that goes along one season at a time, with renewal always in doubt. The season finale there is a slightly odd beast, because ideally it has to both provide a satisfying conclusion to the story of these people if there’s no renewal and leave things open for the next season if it happens.
But I do like to get to end points, places where we can say “OK, that meta-problem is solved and we can go on to other things, we don’t have to go back to firefighting the individual problem manifestations one by one”. It’s a sense of progress, while also saying on a game level that we’ve done with that specific class of adventure and it’s now time for a change. (For example, in my standard investigative weirdness model, the season enemy might be someone who’s causing outbreaks of weirdness – the first few adventures would be fighting those, then there’s progress to seeing a common factor, tracking down the individual, and as a finale the big battle against them. After that “monster attack caused by random weirdness outbreak” won’t be a thing any more, but there’s still weirdness in the world.)
In terms of books I think the typical individual adventure is somewhere between a chapter and a novel in length. The RPG season is somewhere between a novel and a trilogy, depending on how many individual adventures there were and how much progress has been made. And as in the pilot episode (see #10), the season finale and inter-season gap when described as such are a chance to modify characters, both PC and NPC; to say “X has had enough, he’s going to retire and I’ll play concept Y instead”.
Even if there isn’t going to be a delay before the next season starts, just as one can put down book 3 and immediately pick up book 1 of the next trilogy if one wants to. Usually though this is a cue for someone to run something else for a bit, and then a bit later if the group feels like picking up a new season they can do so.