I guess I could always experiment - make a half-hour video that covers the entire topic, then break it into a series of five-minute videos that focus on subtopics. People with a serious issue in the broader topic might watch the longer video, while people on the run might sit through a shorter video.
I think shorter videos might also be a better asset for my web pages. If I have a long article about mammal ecology, rather than embed a long video that covers the same topic, I could embed a short video that reinforces the section on distribution, another short video focusing on mammal diets, etc.