Wherever a user controls the viewed duration of a video ad (e.g. Facebook and Twitter), the ideal video ad duration is one that feels relatively short to the user, which thus increases the chances of the user completing the video ad.
An August 2014 Socialbakers study analyzed 3,000 videos posted to Facebook by 1,000 pages, and found that
1. the average duration of a Facebook video is 44 seconds,
2. completion rate decreases as video duration increases, and
3. videos under 21 seconds long have the best completion rates (70%).
A 2012 study by Wistia analyzed its vast collection of hosted business videos, and found that videos under 1 minute long have an average completion rate of 60%, videos with durations between 1 and 2 minutes have average completion rates of 49%, and so on.
The longer your video ad, the less people will watch its entirety. This is problematic if your main value proposition requires the entire video to be properly conveyed, and also if you hope to effectively harness a post-roll call-to-action that directs users to a deeper brand experience or a digital point-of-purchase. (Depicted below is the Facebook post-roll CTA, available only for videos uploaded directly to Facebook. Wistia also allows post-roll CTAs.)
Boredom and irrelevance are the true drivers of video dropoff. If your 3-minute video ad is extremely interesting and relevant to a user, they’ll keep watching it. However, because the vast majority of advertisements are not significantly interesting or relevant to users, and because users have control in many digital spaces over how long they’ll watch a video ad, brands are best advised to keep the ad as short as possible.
It’s okay if a video ad ends earlier than a user expects if there’s a clear path for interested users to continue learning (e.g. a link to another video in the series) or to purchase (e.g. a link to your digital point-of-purchase).