I have a lot more respect for authors who end their own series versus when the publisher does it due to poor sales. The latter really does make some readers really angry. Some long-standing series really do need to stop, Diana Gabaldon anyone? Gabaldon’s books are thousands upon thousands of pages long.
The protagonists in her books are probably in their 60’s by now or older. What many readers fear is that Gabaldon will kill off her characters when the series end or they will die (the reader) before she ever pens the last novel. My response to that is: how cruel either way you look at it.
How long should a series last? A series should last only as long as the author had originally intended. That doesn’t mean to write 5 additional books just because your sales are strong. I know that sounds crazy but coming from a consumer, it really isn’t. If the story arc is to last only 4 books then stick to it! Also, where is the subtlety? The surprise?
A lot of the time as I’m reading the first book in a new series, I’m being introduced to six brothers and two sisters along with a couple of cousins and all I can do is groan aloud at how many books that will take. I say, you’re no longer writing for the sake of story completion, no, you’re writing for a check.
Flicker pic via theogeo