I read this morning in Nate Silver's new book (recommended by Fred Wilson and published September 2012) "The Signal and the Noise" that the average time a stock was held was 6 years a few decades ago, recently the average is closer to 6 months.
This is one of my favorite charts on the subject, also from NANEX: http://imgur.com/DxWer
Check that out, it gave me a new appreciation for the changes in volume and volatility we're seeing.