This Week's Topic: Series--How Do I Know When One Has Played Out?
To know when a series has gone stale, it helps to understand what makes a series compelling in the first place. Writing a series that readers want to stay with comes down to two things: a) knowing the series's story arc--which is different from the individual book arcs--before penning the first book, and b) how many books it'll take to tell the series's story. Yes, the author should know from the get-go how many wedges they're trying to carve out of one pie complete tale. Each book serves as a plot point of the series. The series plot often runs as the secondary plot of the book; for the sake of pacing and focus, it never dominates the main plot of the book. The series is done once the overarching story is complete.
For example: A classic story structure is three Acts in one book. Trilogies are three Acts told over three books. One could argue a trilogy is a nine-Act story where the pinch-points are the equivalent of the climaxes of the first two books.
Yes, yes there are popular series for which there is no series story arc. Those series are--for all intents and purposes--stand-alone books (self-contained stories) with a repeating protagonist. There is minimal character and world development from book to book, which allows a reader to jump into the series at any point.
Now, to the question of the week: How to know when a series has played out? The most obvious one is when the series story is complete...yet it won't end. Instead of writing a new series, the same cast appears on a wholly different quest. This can work quite well--as long as it's clear in the branding-- until the majority of the cast has developed into OPs (Overpowered Players), the stakes have risen beyond the fantastical, and the plots have no place else to go but into the absurd.
Rattling off TV shows that Jumped the Shark is arguably easier than naming shows that were canceled too soon (aka before the series story was resolved).
Another way to tell when a series is played out is when books within the series are long tangents that do not support the series story--it's being milked. When done a handful of times (or less) as clearly labeled side-stories, the readers understand those books are fan service (stories written purely for the appreciation of the fans {fan services is often smuttier than the original, too brow waggle}). The readers typically love it as long as the side-stories don't delay the conclusion of the initial series storyline, which is why these stories are best released after the conclusion of the series or as a seasonal/holiday bonus. But when the author tries to trick the reader into buying books unrelated to the series plot, then what an author gets is pissed off fans.
Series that run-on too long tend to happen because of fear. Fear from the author that they won't be able to reproduce the "magic" that made that series successful. Fear from the publisher (who may also be the author) that their sales will plummet, thus their revenue will plummet, once the series ends. Nobody wants to end a good thing, but even the best cake rots when kept too long.