The attention span of people watching video on the internet is considerably shorter than say, a video on a television. So if a TV ad is typically :30, or even :60, try keeping it :30 or less. It may also depend on where you are putting this ad. If it's standing alone, or part of another program. On Hulu I believe they stick :10 ads into the programs, which makes sense to me... people tend to click away very quickly if they are become uninterested by an ad.
30 seconds or less, lots of quick cuts if possible, flashy, music is key to at least make it an appealing video to watch, even if they're not interested in the product.
I'd say keep it 15 seconds and under and make it as compelling as possible. Assuming most people are like myself, when on the internet I look for the quickest way to close or skip any advertisement.
I'll agree with 10-15 seconds. It takes time and bandwidth to watch crap you probably don't even care about in the first place, and all you wanted to do was watch a stupid video. What I really hate, are the ones that FORCE you to watch a 60 second spot about whatever, and you can't even start to load the video you wanted until it is done.