I agree with the previous comments. It really depends if you feel like you've fully answered the question. They want to hear your story, but they don't want rambling that doesn't add anything of value. Remember, they are reading through thousands of applications. Interesting and to the point responses are what catches their attention. I would also recommend trying to fulfill half of the word limit, but if you fall short of that and you feel like you've clearly stated your point, then that's okay too 🙂
(I personally wrote about 250-300 words for a 500 word limit response.)