MarketingShfit posts that very short Online Video ads are more effective than longer ones:

"……….insightful analysis on ClickZ about consumer tolerance of ads, which shows that 5 second ads will likely be the way to go.

Consumer irritation grows quickly after 5 seconds, so using short spots in between free videos for Lost or Grey’s Anatomy would be a beneficial mix. Also, the same short ads could be used on TV, as the writer points out that DVR users often rewind to see the last few seconds of ads before a show resumes when skipping past ads. "