​The inevitable question about when to end a series

I’m attached to my characters. All of them.
I’m even attached to the ones that readers write to me about and say “I love every single one of your characters but … .” Guess what? I love the “but,” too. I don’t ever really want to end a series but it’s inevitable and I would rather pick the right time than let something linger longer than it should.
As an author, you walk a fine of trying to make readers happy but doing the story justice. I can’t tell you how many times people have written to me and said that my series should be no more than three books. Each and every series. If it goes longer than three books I’m a hack who is just out for money.
On the flip side, a lot of people write to me that they never want a series to end. They want it to go until the end of time and if I ever stop writing the series they’re going to hate me forever. That’s great in theory, but it’s not so easy (or pragmatic) in practice.
The longer a series goes you risk tipping into “diminishing returns.” What is that? At a certain point you stop sucking in new readers for a series and you start losing them. That happens to everyone.
Now, I’m not someone who pays much attention to diminishing returns. I am lucky enough to be able to write full time and I never end a series before I decide it’s done. I always have an idea of what I want the end of a series to look like, a set story so to speak, but some of the stuff between can be more vague. I try not to focus on the money angle over everything else. That’s why I keep my books at $3.99 and in KU.
Other authors or publishers give a series one or two books and then yank it without an ending if it doesn’t perform. I don’t do that. I always finish everything. I’m a bit of a completionist.
Take Hardy Brothers Security, which I wrote under my pen name. That series didn’t sell well and I still wrote twenty-four books. I was attached to the characters and had a specific story I wanted to tell.
I don’t know anyone else who would’ve written that many books in an underperforming series. That’s simply how I’m built.
Now, when it comes to series like Wicked Witches of the Midwest and Mystic Caravan, I’m honestly not sure how long they’ll run. I know I have quite a few stories still to tell. I “kind of” know how they will end but I have a lot of room to play in between.
My grim reapers, however, will go nine books. That means they end next year. I already know how they’re going to end. In fact, I know what the final line in the book is going to be. That doesn’t mean I won’t revisit the series at some point. That’s not something I just say either. Covenant College folks can attest to that. I had a set five books for that series and then had a fun idea and added six more books in two follow-up trilogies. That could easily happen to Aisling Grimlock. However, right now it will be nine books (although the grim reapers are most likely going to pop up in my Charlie Rhodes series and another series I’ve been working on outlines for) and then there will be a break before anything else is decided. It’s always nice to step back for a little bit, because when I have ideas for new series (or a continuation of an old) it’s usually when I take a step back.
As for Avery Shaw, I keep going back and forth. I believe she’ll go fifteen books but she’s honestly my favorite to write (yes, I know other people hate her, you message me all of the time). She’s essentially me, though, so when you write that you hate her it’s kind of like you’re hating me. Luckily for you, just like Avery Shaw, that only makes me stronger. Anyway, if Avery Shaw goes longer than fifteen books – and that’s honestly up in the air – it will turn into a series where there is only one new entry a year. Like I said, I keep waffling on that one.
Why is this important? Why do you care? Unlike other authors, I have more ideas than I can write. I honestly have outlines done in another twenty series or so because I have so many ideas. Whenever I have an opening and can start a new series I’m excited … until a great war wages inside of me to pick a series to focus on. I just had another idea this past weekend I’m dying to write but I don’t have time.
The thing is, I never want to stop a series because I love the characters. However, I do want to get to know new characters and series going forward, too. It’s honestly a double-edged sword (I have two future series where swords would make regular appearances, by the way). I do give it a great deal of thought, though.
So, what do you think? How long do you think is too long for a series?