Unkle Jacob finally comes out with a good one after a long period of frankly passive advice (in my opinion, of course).

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We’ve
just begun research on the usability of online video and other
multimedia elements on websites. While I’ll surely have many more
guidelines later, for now the main guideline for producing website
video is to keep it short. Typically, Web videos should be less than a
minute long.

The Web is its own medium. We seem doomed to learn this lesson again and again.

  Talking-Head Video Is Boring Online