There’s one answer to this, the publisher’s answer. According to Meg Cabot, most publishers are looking for something under 100,000 words, and teen novels tend to be closer to 55,000.
This is a very neat answer on what’s salable, but it doesn’t actually describe how long a story should be; Long running series are a thing. It’s the lifeblood of comics and TV series alike. Anthology series like Hitchcock Presents, the Twilight Zone and Black Mirror are the exception rather than the rule; The rest are trying to run to 100 episodes on the back of a single cast, and that at the very least means character arcs.
So how do we know when a long-running show should keep going, and how do we know when it’s time to finish with dignity? How long should a series be, whether it be television or manga or fan fiction?
My answer is; As long as it can reasonably continue to escalate.
Now, what do I mean by this? Escalate is a vague word. But really all I mean is; Your story continues to open up more possibilities than it resolves. Either the tension increases, the stakes increase, more strong characters are added…
So long as the visible potential of the world expands faster than it’s closed off, while delivering satisfying payoffs, your story should continue to hold a viewer’s interest.
There’s a balancing act here. You don’t want to burn through too much material too quickly. That can cause burnout for you. As well, the interesting parts of your story are what you explore and flesh out. Expanding your universe too quickly can be overwhelming, and fill your story with irrelevant details.
Not expanding it quickly enough can cause repetition, stagnation. Zombie series. Common examples of this are usually The Walking Dead and the Simpsons, but I’m sure you can think of many sitcoms that shambled well and truly past their expiration date. I hear Big Bang Theory has eleven seasons now?
These stories continue because they’re comfortable, familiar. Its audience enjoys the routine of it too much… until the quality declines too much, and it gets quietly cancelled between seasons.
Some shows appear to balance building the world, anticipation, with meaningful payoff. If this is illusory, a bait-and-switch tactic, it works exactly as long as the payoff seems worth the buildup. If meaningful emotional payoff isn’t given, though, your audience might see through the facade, or grow frustrated.
Story buildup isn’t the same as story building. I mentioned in Pantsing Vs Plotting that Homestuck and Game Of Thrones are examples of series that constantly build tension but don’t have meaningful payoffs, but the truly dubious gold medal has to go to the BBC’s “Sherlock” series. HBomberguy did a fantastic video on how Sherlock did this, and I recommend all two hours of it. It’s comprehensive. But if you don’t have two hours to watch it — again, I highly recommend it — I’ll summarize what I mean: [Spoilers [Duh]]
Sherlock constantly escalated without satisfying payoff. Each season always ends on a cliffhanger, it always ends on a bigger threat, it always ends on next season the stakes are higher. But what pieces does it actually advance?
Sherlock? Watson? No meaningful character growth since season one. It introduces a character, Mary… then kills her off, so as not to risk the established dynamic. Moriarty is alive, then dead, then alive, then dead- Always as bait-and-switch for a larger sense of tension, never meaningfully resolved.
Mycroft? Mycroft is diminished as the series goes on. He goes from a mysterious figure in the British government, implied to be the British government, and devolves into an arrogant, foolish twat in a suit who gets everything wrong. He’s made boring for expanding upon; they drain that well.
So Sherlock implodes after its last season, as everyone realizes what they suspected all along; Nothing’s really changed, nothing has been resolved, but still the show promises that the payoff is just behind one… more… hill.
It’s a useful case study. I rather enjoyed the first two seasons! But it didn’t build its world; it cannibalized it. And it shows why payoff is just as important as buildup to the longevity of a work.
Questions are a bit of a fossil fuel in storytelling; you burn them up to power your story. If you don’t think of enough new ones to explore, you rely on stoking the already-burning coals, and they can’t keep the engine moving forever.
Sherlock wouldn’t run into that problem if, like good BBC mystery series, it actually solved mysteries. Crime serials can carry on with the same cast week after week, for years, and maintain their loyal audience. Why? The characters don’t advance much, nor the world. Yet, they continue to be interesting.
Because as long as there’s still an interesting crime to tell, a new way to explore the premise, the potential is still larger than the shown. Serials of this nature — from cop shows to House — last as long as they can still show a new mystery in a new way.
Soap operas and long running slice-of-lifes can run so long as they can consistently give meaningful payoffs as much as they build up to them. The stories here don’t need so much fuel to keep chugging along, so long as the characters enthrall us; So what’s their limit?
Here we get to the other side of escalation: Too much, too fast, too absurd.
Anime and manga hold fantastic examples for us here, because of how the Japanese publishing industry for them works. A new story every month, popular ones stay and the unpopular ones get three issues to wrap up before cancellation. Like the sitcom problem, manga writers can’t stop as long as they’re popular: Doing so is seen as killing the publisher’s golden goose, and heavily frowned upon, even if it’s for the good of the story.
So we see a lot of stories forced to escalate to absurdity just to keep getting the popular votes every week, and we can learn from that.
Have hard-written rules of your setting been voided by a new character or plot? Are you exploring a mysterious location that was never meant to be given screentime? Are your characters still recognizable held next to their counterparts at the beginning? Is this the eighth time in as many books your protagonist is forced to surpass their limitations? Are you feeling forced to introduce new characters just to break up the existing dynamic, even though they don’t really fit in?
Fatigue can set in. Your story was built with a certain strength of foundation, and it can crack under the weight of having to pile more and more on top of it. Even if, if you manage to keep escalating, keep building the world and pace it properly… at some point, you might just escalate to the point of absurdity.
Where that point is can be different for every author, every story. But sometimes it really is best to cash your chips in and take a risk on a new piece.
Quick summary? A story should expand to fill available space. The story can keep going while there’s still space to be filled, but it should try to keep expanding as well. When there’s nowhere left for it to grow, when the world is filled, it might be time to wrap it up.
 A friend editing this pointed out the brilliance of the comic Dr McNinja, a long-running series by the brilliant Chris Hastings. Dr McNinja continued to build its world, but every revelation made the world make more sense. Then, when it had no more mysteries left to explain, no more absurdities left to resolve… it finished on an extremely satisfying ending.