This summer Philip guo, an assistant professor of Computer Science at the University of Rochester, spend time at EdX as visiting research scientist.
In his research he looked at video usage, obtained from analyses of a few edX math and science courses. According to his findings the optimal video length is 6 minutes or shorter.
If you look at the classroom we all know that the attention span of a student is not 45 minutes, but what the exact attention span is, is not clear. Some say it is variable, others say it is 10-15 minutes.
The general rule is shorter is better as shown in this blog post.