Online video makes it easy to publish a ninety-minute lecture. Two lines of research suggest that is rarely the best way to teach with it: attention fades across long videos, and short questions between segments bring it back.
Attention by the numbers
Guo, Kim and Rubin (2014) analysed 6.9 million video-watching sessions across four edX courses. The clearest finding concerned length. Learners typically watched videos under about six minutes to the end. As videos got longer, the share watched fell: roughly half of a 9 to 12 minute video, and roughly a fifth of a 12 to 40 minute one. Across lengths, the median time learners spent watching was at most about six minutes.