How long should a MOOC video be?

At last the answer to a question that I've always known the answer to in an intuitive sense but I've never had the any real evidence to justify my assertion.

How Video Production Affects Student Engagement: An Empirical Study of MOOC Videos (Philip J. Guo
MIT CSAIL / University of Rochester, Juho Kim MIT CSAIL, Rob Rubin edX)

The University of the Future

The University of the Future.  The BBC have launched this programme that analyses the MOOC phenomenon.  Well worth a listen with some quite startling highlights:

1. Some academics can make themselves into global superstars, taking their teaching to an audience that they could only have dreamed of.
2. Watch out for UDACITY.  The early MOOC rush created some lousy products.  UDACITY seem the most likely to be innovative in the pedagogy of MOOCs.
3. MOOCs are irresistible.  The tide has already turned, undoubtedly some universities may suffer unless they adapt to a new paradigm.
4. In 50 years time will there be only 10 Higher Education institutions?

(http://www.bbc.co.uk/programmes/b03wpf59)