One of the consistent messages from Jakob Nielsen has been the speed at which users spend on content on the web. (See How Little Do Users Read.)
His most recent contribution is to remind us how different TV is from the Web. He reminds us that web users have many choices and much control over what they view and how long they view it. Likewise, there are many additional features in addition to just passively viewing content. He suggests that web users make decisions every 10-120 seconds compared to 30-120 minutes on TV.
He suggests that web video viewers are in the 2-10 minute range.