The best video ads are 15-sec long

Want to make the best promotional video? 15-sec spots work best as pre-roll ads, 30-sec work best as click-to-play.

Our strategy is to make the best content as easily digestible and distributable as possible. This means, don't pay $30-40k for a 3 minute video most people will only watch a few seconds of before realizing "oh, what? it's 3 minutes long? forget it". Instead divide it up into small chunks which can be used as pre-roll spots, click-to-play ads, and standalone videos. This way you can have multipurpose ad content, and you can build out your YouTube channel, which will have the added benefit of helping your organic rankings.

http://www.iab.net/insights_research/947954/dv_effectiveness