DH asked me that question the other night in bed. I'd finished the first draft of
A Modicum of Truth Wednesday evening, and I mentioned I got the inspiration for the starting scene of what will be Book 4 in the Justice series.
I know a lot of indie writers are very concerned about the optimal length of a series. This is my opinion, and mine alone:
As long as I'm having fun writing a series, I'll keep going.
But there's another thing that's personal to me as well:
I go in knowing how the series will end.
Seriously, I have to know where my characters end up before I start writing. I may not know how they get from Point A to Point Z, but I start a project knowing exactly where Point A and Point Z are for the primary character.
I'd give specific examples from my work, but I don't want to spoil things for people who haven't read my one completed series, nor do I want to spoil things for the books that aren't out yet.
So let's use a best-selling author: J.K. Rowling.
Her character, Harry Potter, has a traditional hero's journey. He beats the bad guy who's been out to kill him since he was born. Ms. Rowling does this through the typical years of adolescence in a U.K.-style boarding school, from age 11 through age 17, i.e. seven books.
Each book had a standalone story while each volume carried the primary story forward. Even better, the ending is a call back to the beginning. Rowling had a final scene that many thought unnecessary, but was in fact quite essential. The book ends with Harry's son Severus on his way to Hogwarts for the first time. It's shows everything in their world is okay and Severus won't face the same terror and danger Harry had.
The extended storyline is the model I prefer, but it is a personal preference because I need an ending, both as a reader and a writer. And because of the ending, you would have a fairly good idea how many books will be in the series.
That doesn't mean there's a magic number. Rowling planned seven books in her Harry Potter series. I planned nine books in the Bloodlines series.
That doesn't mean you can't add or subtract if things are working/not working. I added a few shorter working to the Bloodlines series because I was having too much fun with the characters.
On the other hand, you could have a series whose main character doesn't really change over the course of the books. Each book is therefore essentially a standalone story. Readers can jump in any time.
The late Sue Grafton's Alphabet mystery series is a good example. Private investigator Kinsey Milhouse changes very little through the course of twenty-five books. Some readers like that consistency. And there's nothing to stop a writer from going for ten, twenty-five, or even an hundred books, a la Nancy Drew and the Hardy Boys.
A major problem to watch out for--some writers keep going because the money is so good they (and/or their publisher) are afraid to stop. As much as I loved Charlaine Harris's Southern Vampire Mysteries (aka the Sookie Stackhouse series), I hate to say this, but it was pretty obvious Ms. Harris lost interest in the series around Book Nine, which was released in 2010.
However, in 2010, the HBO TV series based on her novels,
True Blood, was at its height of popularity. Both HBO and her publisher wanted to maximize profits, and they offered her a pretty penny to keep going.
But Ms. Harris lost track of the overarching plot (who Sookie would choose as her life partner), and she didn't plant enough clues of who Sookie's ultimate paramour would be. Readers were sorely disappointed about how the series ended.
Even though HBO had deviated from the main plot of the books, their ending wasn't received any better.
To me, this is one of the best reasons to have a particular ending in mind if you're writing an overlying plot.
But ultimately, the decision of how to handle your series is up to you the writer. Just remember the Golden Rule and treat your readers how you would want to be treated in that position.