You want to keep your video to 60 seconds or less because a significant chunk of people will watch about a minute of video before they drop off.
Video quality is a factor to consider. Obviously the better your video, the more likely someone will watch it longer or all the way through. That means it's important to write a good script that is performed well and has good visual production values.
Video completion rates can vary based on whether the video is used in email marketing, PPC, at a tradeshow or as general website content.
If you cannot make a shorter video and still cover everything, then try to get the most powerful points across in the first 30-60 seconds. Another tactic is to use with longer video, after it's produced, to create smaller video segments that you can deploy in your marketing programs and social media.
Longer videos can still produce good results even if the completion rate is NOT very high. I see it happening in AdWords campaigns. We have video landing pages with videos that are 90 seconds or longer that achieve good or even great conversion rates. Keep in mind the video is only a part of the total conversion funnel in a well managed AdWords campaign so other factors may be affecting conversion rates.
Something else to think about when getting lower completion rates for longer videos is that you are still getting completion rates. And it may be that those 8-20% of people that take the time to watch your longer video are the ones you are really trying to reach. Food for thought when deciding on video length.