Many artists and bands will post a teaser video of their album before it's released, or as a brief sampler afterwards. The problem is that there's several schools of thought on how long the teaser should be.
One school has it that shortest is best, since YouTube attention span is around 8 seconds. The second school says that you should make it as long as necessary to get the point across, even up to several minutes long.
It turns out that neither may be right. According to a study done by Think With Google, good old fashioned 30 second ads performed far better than either 15 or 60 second ones when it came to people viewing it all the way through. This allowed the viewer to get more detail about the product without the dreaded viewer fatigue.
This contradicts what happens on television, where 15 second ads are up to 75% more effective (and cheaper) than their longer counterparts. No surprise since online advertising has proven to be substantially different because of its random access nature.
The bottom line is that 30 seconds is a sweet spot in that it's neither too long nor too short, giving you enough time to get the point across while the viewer doesn't feel like she has to sit through too much information.