There’s no simple answer to how long a particular video should be.
It depends on a number of things… For example, how much have you got to say? What video, stills or graphics do you have to illustrate what you’re saying.
If you’re trying to reach younger viewers, you might want to keep your videos shorter - especially if they’re going to be viewed on mobile devices.
If you’re targeting an academic or highly intelligent audience, you should probably opt for a longer duration.
There is a place for both short-form and and long-form videos on the Internet, and now we’re seeing the emergence of micro-videos as well.
Remember, people are busy, and attention spans are shorter than they were. So, if in doubt, err on the side of brevity.
Generally speaking, the optimum length for a video is somewhere between two and three minutes.