The answer to this question often varies by writer. Some writers won’t show their work to anyone until they get through several full drafts. Others are eager for feedback even before they have written the end of the story. Listen to your own writerly needs. If you are spinning your wheels, a sensitive reader might provide just the traction you need to move forward. If your ideas on a current project feel precarious, you might give yourself the time you need to figure things out for yourself before getting outside input.
A good general rule of thumb is to wait until you have taken the draft as far as you can on your own. Your draft may seem very strong, or it may have problems that you can identify, but don’t know how to solve. Often, writers get so anchored in their own work that they lose objectivity. A careful reader can help you see the strengths, flaws and possibilities that you’re not seeing on your own.
It is also useful to consider who you show your work to. Other writers can make great, informed readers. People who don’t write but who read regularly can be valuable readers, too. Non-writers might not use terms like “rising action” and “point of view strategy,” but they can share their emotional responses to the characters and note where their attention was captivated and where it flagged. And you want to make sure you choose someone who is both supportive and honest. You’re not looking for hollow praise, after all. You want a thoughtful look at how well your intentions made it onto the page.