As a relatively new mystery author, the last thing I have to worry about is carrying my series on for too long. I should only hope to have this worry some day.
But I've talked to a couple of author friends who DO have this problem (?) and asked them, "When do you know your series has run its course?"
Short of their publisher telling them it's time to give it a rest, they have taken the problem of a sleuth growing too old to chase down criminals and taken them back a step in time to a prequel. What this basically means is, the retired sheriff is back to being a rookie deputy, the veteran special investigator is back to being a fresh-out-of-the-academy patrol officer. At this point, in the Black Horse Campground series, I've kept the main characters at an age where chasing down criminals is still as believable as being involved in a romantic triangle, so I have a lot of time to work with (although it occurs to me that Agatha Christie had no problem with having detectives that were already at or past retirement age when they adventured through several dozen books!)
The problem, of course, lies in the fact that after so many books and having grown with the character over a span of many years, many readers love taking a voyeuristic look at the character's private life before they made themselves into the character they love now... and that makes a monumental task for the writer.
How much of their character's private life do they reveal? How far back do they go? And what, after all this time, is there new to introduce about a character many readers feel they know as a personal friend? And worst of all... how many times can you pull that off?
The two author friends I mentioned before have taken different tacks. One did go back to a prequel, and then deftly shifted the series to another (younger) character's point of view. The other is still on the fence about whether to go this route or allow their character to exit-stage-left gracefully and leave the readers with fond memories of the journey shared with this series.
Whatever route an author chooses, one thing is certain: if they have successfully created characters with whom readers closely identify and come to care about, it's never easy to shift away from the here and now. Many readers "grow up" with the character during the course of a series and sometimes it's just as hard to go back to the time when they were still finding themselves as it is to say it's time to take a bow and leave the stage to a younger protagonist.
What are your thoughts about the length of a series and how a character ages?