When to end a series

I find there is always more to write for my series, but I am not sure I should continue the story when I have finished one arc.
My story has come to a conclusion, but I can still write about the girls time in high school and college. The international intrigue is over though.
How do you decide when the journey of your characters is over?

When the characters move away from the scene of the story line or when one or more becomes deceased.

When one has a hard time connecting the newer stories to the past stories I generally end mine. I do find with rare exceptions that any series over 4 or 5 get the reads or the comments that one would hope. It seems like a lot of readers do not want to go way back and start at the beginning. However these are just my thoughts and I encourage you to do what you feel is best and want to do.

CS

One way of looking at it is if each series is a separate novella or novel. Once one story is over, you can write another one set in the same universe, with the same characters, but assuming that they haven't read the first series - and might be reading them out of order.

In short, include some paragraphs covering what happened before. Reintroduce characters, assuming that the reader is meeting them for the first. Maybe write a series from another character's point of view.

Then again, if the series is episodic, it can go into hundreds of chapters. As long as you like writing it and people like reading it, write on and on. It's not like you're charging by word.

I'm generally in favour of leaving the reader wanting more rather than carrying on too long, sometimes to the extent of deliberately leaving the outcome ambiguous so that the reader can imagine their own scenario.

GG is absolutely right though. Each series is a separate story, it's only a series because it's broken up in convenient sized chunks. It's best if it has its own plot arc. If you like the characters or they have more adventures, then by all means write another story about them.

There's something a bit daunting about a story which states up front that it has 60 or 70 chapters, and breaking it into smaller bits gives the reader an easy place to pause.

1) End the series before it runs out of steam and starts to limp its way to the finish line. Finish the arch involving your main character/characters and move on. If you have developed a secondary character who has proven popular you must give resolution to that character's story as well (Everyone wants to know that the "mean girl" got what was coming to her).
2) Don't "jump the shark" It will be tempting to make each spanking longer, harder more preposterous than the last one. Don't do that. Everyone in town does not have to spank our heroine in order for the story to be satisfying.
3) Finish before you start introducing more characters that will be hard to keep track of and just serve as filler between plot developments that the reader actually cares about.
4)Unless you're careful at crafting your tale the longer the series is the more convoluted the story becomes. Better to walk away having KISS (Keep It Simple Stupid) as a working motto.
5) There are a lot of good reasons to keep a story going through even numerous episodes. But starting over mid-way through the series with a new setting and new characters is not one of them.
6) If you are having difficulty staying faithful to the universe you created, time to call it quits. Your characters need to have the same qualities and points-of-view in chapter 15 as they did in chapter one. Sure we accept character development and growth but not someone arguing an opposite view point five days after taking the other side in a discussion or argument.
7) If you are getting tired of your own story and stop paying attention to detail it's time to write THE END. I read a story in the library that suddenly gave the family a different last name. It was pretty jarring.

Sweetspots's list is very sensible.
I would just add that if you are bored with the series or finding it difficult to continue then it's usually time to finish.
However sometimes I take a long break and return refreshed if I think the series still has legs.
If you want to publish as a novella, it gets a bit more difficult as you need 25 to 30K words.