Like so many questions about writing, this one is difficult to answer. Technically speaking, your story is never really done. There are always ways to change it and improve it. However, at some point, you have to make the decision that the story is good enough. You have to say that you aren’t going to make any more changes.
This can be difficult. It can be hard to know if the story is “good enough.” After all, we are our own worst critics, so we never believe that our work is “good.”
One thing that might help is to have others read your story. Start with an editor (or two), so you can fix any content or character problems. Then, you’ll need to make sure your grammar and sentence structure are correct. After your story has gone through a few rounds of editing with a professional editor, then you can send it to beta readers.
Beta readers play an incredibly important role in the writing process, and it’s a job that shouldn’t be taken lightly. Like an editor, beta readers give you suggestions on how to improve your story. In most cases, beta readers have not been trained as editors, but as readers, they know what works in a story and what doesn’t.
The goal of your story should be to invoke some type of emotional response in your reader. To do this, they have to understand what your story is about, what emotions you are trying to convey, and react to your characters in either a good or bad way (not all characters have to be likable). If you haven’t accomplished this goal, then you’ve probably failed in your writing attempt. That means you probably need to go back and rework your story.
Editors and beta readers are great resources for helping you achieve your writing goal. They can point out flaws and plot holes, along with ways to improve them. However, keep in mind that everyone who reads your book will bring their own experiences and biases to the work. No two people will react to it the same way—and that can further complicate if you view your story as finished or not.
Every reader is unique, and even if readers aren’t reacting the same way to your story, if they are still reacting, you’ve probably accomplished your goal. Some will love it, and some will hate it, but as long as they aren’t saying, “Huh? I don’t get it,” then you’re on the right track. (Side note: there still may be a few readers who are confused by your story, but as long as the majority of them aren’t having this issues, you’ll be just fine!)
In the end, no matter what an editor or a beta reader says, you are still the creator of the story, you get final say in what gets changed and what doesn’t. Editors and beta readers will have different ideas of how a scene should play out or about a character’s personality, but you get to make the final determination if you take their suggestions or not.
Writing is both a simple and complicated process. There are rules, but there aren’t any rules. When it comes to creating, most of the time you have to follow your heart and your head to determine when the story is done.