Good question, and difficult to answer.
I would guess it all depends on how long you need to get your story/point across. What concerns me about that is two things. 1, I personally am typically lazy to read a long post, but I think the fear of missing out on a really good post or subject encourages me to read. 2, with the invention of twitter, and its use, I see that more and more people’s attention span is being shortened, as I see this in myself. My mind couldn’t focus too long on a subject or topic because there was another 140 character post to be read, and another, and another.

I think, your posts should be as long or as short as you need them to be.