My best performing pieces are almost always in the 7 - 10 minute range. I think it's a moveable feast depending on what your subject matter and style happen to be. I write a lot of long commentary and opinion and people seem to stick around then drop off around 8 - 10 minutes. If I hit 11 - 12 minutes, I start to cut it down. 8 is the sweet spot for me. :o)