What is the optimal length for a video?
Social Bakers, a provider of social media metrics, conducted a study of 3000 videos on 1000 web pages. They measured the completion rates for videos posted to Facebook and concluded that videos running 21 seconds long had the best chance of being viewed all the way through.
The study concluded that approximately 21 seconds is the optimal run time of a video that viewers will watch without leaving.
You can see how the rate of sticking with a video declines as the running time of the video increases.
See the study here