It is a double edged sword, but the longer the better (up to a limit). 1. If people are watching through 95% of a 2min video ad, it shows insane intent. A 10 second video is almost statistically irrelevant - you might as well use an image. 2. You get solid intent segregation as opposed to a short ad. I know 95% of a 1-2min VV is going to be warm. I can retarget this group and have a high likelihood of sales. For 50%-75% VV of a 1-2mins video, I would probably nurture them a little bit more before asking for a purchase. You wouldn't nearly have enough data for intent segregation on a 10-20s video. 3. If people lose interest, it's not the video's fault - It's just bad script/copywriting and/or sub-par Creatives. Catching people's interest is one thing (good headlines/thumbnails), holding people's interest is another. Don't blame the format if the content is bad.