They vary depending on what's involved. But I don't think its the length of the video that is important - its the usefulness of the content, the quality of the instruction, and whether you can take something away from easily it that has some application to your tennis.
Obviously, you don't want something too short, but equally you don't want something too long either because it becomes tedious and the message gets muddled.
You want him to convey the concept clearly, demonstrate it perhaps a couple of times, perhaps from a couple of different angles where necessary (I like the fact he takes time in the course to flip the videos so that he has a 'righty' swing, since most players are right handed), and then you want to be able to get out on the court and get a feel for whatever it is that he's suggesting.
Have a look at the suggested shuffle step for the 1HB. You don't need a long video for that because he gets to the point straight away:
http://www.jeffsalzensteintennis.com/1-hand-backhand-tennis-footwork/