I am great at summaries and make easy to read short articles. People like my writing style and filtered content.
It really depends, I think you need to write until you feel you have covered your topic to the level at which your readers expect you to do, no more and no less. Some articles flow easily and others don't, but with experience you'll learn how to partially overtake these kinds of obstacles.
I usually target between 400 and 600 words, but I'm a very long-winded writer and sometimes it becomes difficult for me to know when to shut up. I have to constantly keep track of where my word count is to keep my runaway brain in check. Otherwise, I just keep going, and going, and going.
Sometimes it seems that I can easily write a very long article in no time at all, and other times I seem to struggle with writing a small article.