If you focus on cheat applications that pay (do they pay?) for watching videos on Yotube, this is 40 seconds. It is in 40 seconds of viewing that the program gives you points.
The work of the view reading algorithm is shrouded in mystery, but one of the legends says that when watching more than 40% of the video's timing, the view falls into the general collection of views. But this legend is already 10 years old. YouTube is changing its algorithms so fast that even the exact answer to this question today - tomorrow will no longer be relevant. And all this because of the Russian hackers, of course.
One download counts for one video view (re-viewing is considered re-download).
YouTube does not really disclose all of its algorithms, but emphasizes in the FAQ that the view counter lags behind the real one and periodically freezes. as views are analyzed and validated (to exclude cheating by clumsy bots). YouTube itself says that new videos are being studied especially closely (according to data on the Internet, such a fate awaits all videos with 300+ views).
Considering. Since YouTube also provides information about the duration of the view, it is unlikely that you need to watch the entire video to increase the view count.
In fact, this is a rather complex topic, because if the algorithm for accounting for views were reliably known, then every fifth "blogger" would increase views.
It is known for sure that 1 view counts as a moment when the video has been fully watched. However, this is an extreme case.
In general, we can assume that views are counted after viewing a certain percentage of the total video length, but this percentage may depend on several factors, including the genre of the video, the average length video on the channel, the use of Adsense (advertising in the video) - but no one can say for sure about this (except Google engineers, but the NDA prevents them).
And yes, if you try to watch videos too often, after a certain the number of views (before it was 301 views - the threshold after which verification of all views began) you will not be able to add more than 2-5 views.
In general, YouTube algorithms are a very dark topic, and every day things just get more confusing.