Dr. Philip Guo, an assistant professor of Computer Science at the University of Rochester, reports on a study of some online edX math and science courses.

If you have been thinking about flipping your class by making a long video lecture, think again. He says:

The optimal video length is 6 minutes or shorter -- students watched most of the way through these short videos. In fact, the average engagement time of any video maxes out at 6 minutes, regardless of its length. And engagement times decrease as videos lengthen: For instance, on average students spent around 3 minutes on videos that are longer than 12 minutes, which means that they engaged with less than a quarter of the content. Finally, certificate-earning students engaged more with videos, presumably because they had greater motivation to learn the material. (These findings appeared in a recent Wall Street Journal article, An Early Report Card on Massive Open Online Courses and its accompanying infographic.)

The take-home message for instructors is that, to maximize student engagement, they should work with instructional designers and video producers to break up their lectures into small, bite-sized pieces.

See the graph and read more:

https://www.edx.org/blog/optimal-video-length-student/1239