What is your target? Let's say how many pages or words? I think many writers either don't know when to stop or run out of things to write. My humble advice is
1. Plan about a message or theme that you really want to convey in this story.
2. When you are stuck, revisit if you have already conveyed the message in the story, if not proceed to conveying the message. If yes, think of another message or simply wrap up the story.
I usually stop when I realize I conveyed my center message. Many times, I figured I in fact conveyed my message in the first couple pages/chapters, the rest are just examples or elaborations. Sometimes I would rather stop wisely and have a complete story, instead of leaving story without an end:)
So revisit your thoughts and hope this helps!
Note: I am not telling you to stop and keep your story short. A story can have several messages, but be sure you check it so that you can make sure you have conveyed it and move on to the next one!