A study by content delivery network Limelight Networks of video ads served during the first half of 2010 found that 15-second ads were more than 20 percentage points more likely to be viewed to completion than 30-second ads.

But examining how ads of different lengths are viewed with content of different lengths gives a fuller picture of video ad tolerance. Ad management technology provider FreeWheel found that 15-second and 30-second ads were completed at almost identical—and very high—rates when supported by long-form content. Mid-length content also saw increased completion rates compared to short-form content, for both longer and shorter ads.

Ad placement also had a significant effect. Mid-roll ads, which typically cannot be skipped and appear when viewers are already invested in watching content, were viewed to completion more than 90% of the time between Q1 2010 and Q1 2011. Completion rates for pre-roll and post-roll ads were lower, and appeared to gradually fall during the study's timeframe.

FreeWheel reported that many publishes are already aligning content with ad lengths that make sense and produce the most views. Longer ads were five times more likely to be placed with long-form content, while most 15-second ads appeared with short-form content. And the high performance of mid-roll ads has helped boost volume for the format by 30% between Q4 2010 and Q1 2011, as more long-form content comes online to support them.