Sustained attention lasts about ten minutes (Evidence from the psychologist Susan M Weinschenk)
"People can take a short break and then start over with another 7- to 10-minute
period, but 7 to 10 minutes is about as long as we can pay attention to any one task.
If you’re designing a Web site, you’re probably designing pages that someone views
for a lot less than seven minutes. You’re assuming that someone comes to the page,
looks for a link, and clicks on it. But sometimes you might be adding in other media,
such as audio or video. These media are subject to the 7 to 10 minute rule. TED videos
are typically 20 minutes long, so they’re going over the limit (although they have some
of the world’s greatest speakers, and so might be able to stretch it).
Assume that you have at most 7 to 10 minutes of a person’s attention.
If you must hold attention longer than 7 to 10 minutes, introduce novel information or a
Though it speaks about the rule, no common man knows about it, its a psychological factor. So keeping the length of the video is
informative, accommodating & valuable such as people can choose to view,as videos are more likeable.
The Caveat: If you are suppose to show an "Ad" on a webpage, people firstly will have to know its an ad (By way of location,purpose and content) and they also presume and knows it will end sooner than later (going by the rule of 7 mins). This is automated since you always expect the message will be running only for very short duration. This sets the mental readiness. So here I would not want to show the length of the video. Same as in case you want to know "What this tool or software is for?" and which will eventually tell you the story in a short time (going by the rule of 7 mins). Since all these are Selling and Pitching areas.
So its a known territory vs unknown territory that the people need to adapt to. As you said TV Episodes are known territory and do not expect the length of the video intact, but what if the user is new to such episodes (since he is from a different country) - then it becomes a valuable piece of information. Ideal location where length of the video is not recommended is in "business selling" scenarios - whereas its required to address and be empathetic to the user for all other informative and entertainment scenarios.