The latest comScore numbers for online video consumption in the U.S. are out today for the month of June, showing once again that Google Sites, primarily YouTube that is, dominate the rankings with a whopping 2.3 billion (with a B) viewing sessions last month. That’s out of a total market of 6.3 billion viewing sessions, the first time ever that comScore numbers have surpassed the 6 billion mark.

The runners-up are the same as previous months, with music video powerhouse VEVO the next highest with 399 million viewing sessions, followed by Yahoo, Microsoft and Viacom sites.

comScore defines a viewing session as “a period of time with continuous video viewing followed by a 30-minute period of video inactivity,” meaning their data conservatively undercount actual views reported from the sites themselves. How many times have you headed over to YouTube for example, and only watch one video per session?

There’s always been one number buried deep in the comScore releases that we’ve always paid attention as it creeps up month over month. That’s the online video duration stat, that measures the average time of an online video. For June 2011 that number hit 5.4 minutes, nearly double what it was a few years back. To better visualize this move, we whipped up a chart of this data over time, going back to December 2007 when it was just 2.7 minutes.

Online Video Duration 2007-2011:

It’s been a steady ride upwards since web series started pouring onto the online video landscape. And while speaking at conferences, I’ve been know to cite the so-called “Hulu Effect”—on-demand consumption of longer-form TV episodes—as the main driver here, it’s probably just as much a YouTube Effect as anything else.

Hulu first launched to the public in March of 2008, flooding the internet with high-quality long-form video content, but it still remains in ninth position with just 156.9 million viewing sessions last month. So as online video networks from YouTube to Crackle to Hulu shift their appetites towards longer-form original content, we expect this number to jump even further.

If the same trajectory were to be taken forward a few years, which is probably a conservative estimate given the current market, we’d expect to see average online video duration at 10.4 minutes by 2014. Time to get that living room TV hooked up to the internet guys.