Well, one sign that it's time to end your story is when there's no more tension or conflict left to explore that would keep the readers engaged. Also, if you feel like you're starting to repeat yourself or the story is getting stale, that might be a cue to wrap it up. Another factor could be if you've achieved the overall theme or message you wanted to convey.
It's a tricky one. Usually, you should end your story when all the main plot points have been resolved and the characters have reached a significant point of growth or closure.