YouTube videos have views. How long do I have to watch a video for it to count as a view?
Till the end. I don't remember the source, but I read it somewhere in YouTube itself.
Update: According to this page in YouTube support, it's not that simple.
According to YouTube's FAQ on May 23, 2012:
I would assume they don't count views as soon as you load the video, since that would make it too easy to artificially inflate view counts. Further than that, it's difficult to say.
protected by Community♦ Jan 16 '13 at 6:07
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