In short, brevity is best!
Advertisers who buy online post-roll ads may want to read this. Online video distribution/measurement service Tube Mogul studied the issue of follow thorough, measuring what percentage of videos clicked on are actually watched until the end. A sample of 188,055 videos on 6 top sites (YouTube was not one of them) found that 10.39% of viewers clicked away after just 10 seconds and that over half of the audience left after one minute.
The longer the video, the quicker erosion; all but 16.62% of the audience left the building by the end of a 3-minute clip. Tube Mogul notes that the research indicates that overlay ads should also be placed as early as possible within the video stream.
Audience Attention Span
Video length % of Audience who watched the entire video
10 sec. 89.61%
20 sec. 80.41%
30 sec. 66.16%
60 sec. 46.44%
2 min. 23.71%
3 min. 16.62%
5 min. 9.42%
Source: Tube Mogul & Cynposis