I am not sure it will be a specific number of words as much as the "feel" of it. It is pretty easy to tell when someone put some quality together even if the post is a bit shorter.

That said, I think you are in the range. 500-750 will likely be the norm with some going beyond there. My morning post is usually 1,000-1,250.

We could also see some value added to the post with charts and graphs so one with less words, say towards your 500 level, but with applicable charts could have more value than a post with 750 words.

Interesting question.