Ask HN: What is the optimum length of an explainer video?

I am working on a script for an explainer video for my product http://www.skillendar.com. To make it funny and more interesting I have to add some unrelated content which in turn is increasing the length of the video to about 3 minutes. Do you think I should restrict the length to 90 seconds in order to retain the viewer's attention? This would mean that I would need to compromise on the whole story line.