Question: How long should an online video be?

Answer: As long as it needs to be.

If your goal is to go "viral," then shorter is better. If you look at the viral hits on Google and YouTube, many are under one minute long. 

Why?

Is it because modern living has reduced people's attention spans to fruit fly proportions? No. It's because the shorter the video, the higher the odds that  viewers will watch to the end and push the "share" button.  It's the pushing of the "share" button that makes a video viral. That's the whole game. Of course, the video has to be a "wow!" to large numbers of viewers to stimulate this response.

But before we shout "hallelujeh" for viral videos, let's take a look at some of today's winners on Google Video
and their suitability as advertising vehicles:

1. "White and Nerdy" - a professionally produced music video
2. "Sex Accident" - less than zero
3. "PS3 vs. Wii" -  a professionally produced ad
4. "Octopus escaping through one inch hole" - dubious at best
5. "Fitness" - less than zero
6. "The Birth" - a comedy skit - dubious at best
7. "Guy pwned by girl" - less than zero
8. "Best penalty ever" - weak maybe
9. "Stars are Blind" - zero
10. "Quarters" - a professionally produced ad

Three of the winners came from professionals and were made to promote a specific product.

Four have no advertising value and at all and three of them would send any sane advertiser running in horror. One is a very weak maybe and the other two are of dubious value at best.

YouTube et. al. could theoretically run an ad before or after the three winners, but that would be strange because they are already advertisements for something.

The remaining seven range from incredibly weak advertising vehicles to postively radioactive in their unsuitability.

So tell me again why we as advertisers are so excited about viral videos?

Back to the question that started all this: How long should your online videos should be?

As long as it takes to tell your action-motivating story to your targeted prospect. If someone is interested (i.e. a potential customer) they will take in all the relevant information you have to offer. Within my own sphere of interest, I have no qualms about watching a 20, 30, even 60+ minute video online, but if something's not suitable for me, 7 seconds is too long and I'm not going to push the "share" button.