TubeMogul recently measured 23 million streams on the six top video sites over a two-week period and blogged about here. And, after clicking play, viewers only watch to the end of 5-minute long Web videos about 10% of the time.

TubeMogul draws two conclusions:

1. Post-roll ads are of limited effectiveness. A three minute video that has a post-roll ad in the final seconds, for example, will only be viewed by 16.62% of the initial audience, on average.
2. Overlay ads should be displayed as early as possible in a video. On YouTube, where most overlay ads appear at about 10 seconds in, 10.39% of a video’s initial viewers are not likely seeing the ad.

I would add a third: keep those videos short!