I disagree with people saying that simply reducing the time of your video will help audience retention. Sure, people don't want to watch a feature film, but you can easily engage an audience for over 5 minutes. My most watched video is over 7minutes long with a nearly 60% retention rate (which I think is pretty good).
Moral of the story is to be engaging. I'm still learning this lesson myself, but I certainly haven't given some arbitrary number of minutes that I have to abide by