The article seems to recommend that lectures be 20 minutes in length, but my guess is that 3-5 minute audio lectures would be more appropriate. In general, it is important to note that there is little data on this issue so all of us are just guessing.
Dalton A. Kehoe, an associate professor of communication studies at York University, in Toronto, has for decades won teaching awards and praise for his lectures. So when he was asked to do his first online course, a couple of years ago, he was excited to head into a studio to capture his 50-minute talks on video.
When the recordings went online, however, they were anything but hits. The main complaint: They were much too long.
"It was the most extremely boring thing my students had ever seen," Mr. Kehoe acknowledges. His course evaluations, usually glowing, grew dismal.
The idea that long lectures are boring should give pause to the efforts to stream classes and do webcasting of lectures given in lecture halls. This stuff is likely to be very deadly.
There are a lot of things that should be learned from radio and television about audio and video. For example, most "talk" radio and television includes multiple voices. There are many interview shows that use "dialogue" rather than "monologue" to discuss serious ideas in-depth. This may a much better format for lectures that the typical person standing at the podium or solitarily recording a lecture.
This does not mean as some of have suggested that short lectures result in "short" ideas being discussed. It means that ideas are presented in short segments with clear paths to the next idea or an activity that allows the student to explore the idea in an exercise, discussion, and so forth. This is really just good teaching. The long lecture was never the best way to teach in the first place.