There is much discussion in online education about the “15-minute rule”: that content longer than 15 minutes will lose students’ attention. Part of this is because of the intrinsic pain of watching videos on a computer. But part is rooted in classroom observations, that students in lectures tend to become distracted and lose their attention for a lecturer after some period of time. Interestingly, the education research shows that this is more complicated. For example, Karen Wilson and James Korn
It is clear that students' attention does vary during lectures, but the literature does not support the perpetuation of the 10- to 15-min attention estimate. Perhaps the only valid use of this parameter is as a rhetorical device to encourage teachers to develop ways to maintain student interest in the classroom. If psychologists and other educators continue to promote such a parameter as an empirically based estimate, they need to support it with more controlled research. Beyond that, teachers must do as much as possible to increase students' motivation to pay attention as well as try to understand what students are really thinking about during class.
Probably the most useful bit is this:
The information processing that occurs during classroom tasks resembles a large working memory task (D. J. LaVoie, personal communication, March 21, 2005). Students receive information from the instructor and must hold the information long enough to record it in their notes or do whatever else they need to do with it. Whether students will be able to maintain their attention in class depends on their working memory capacity as well as their motivation and arousal (Pashler, 1998).
This suggests that instructors should provide multiple cues to promote effective use of working memory during their classroom presentations. Specific callouts to reading materials – preferably search terms in an ebook – might be helpful. And visualizations ought to be consistent between lecture and readings, so that students can calibrate their note taking.
As for online presentations, I still think that short is better. But more to the point, they should also be calibrated to other material, such as online readings and quizzes, so that note taking will be more productive.