Youtube – How long do I have to watch a YouTube video for it to count as a view

Your views are not updated in real time so there is no way to know exactly when somebody actually seen your video. As for notifications I don't think that there is a way (at least not directly from YouTube) because it would be quite annoying to receive notifications for viral videos.

This is just speculation, but I would expect that they'll be counting all page views from the same IP address with a certain period of time as one view, obviously it's not as simple as this as a lot of people share IP addresses, there will probably be cookies involved as well. This will stop people artificially inflating their view counts by repeatedly refreshing the page from a single computer.

I don't know how long the time period would be - it may in fact be effectively forever so if you went back and viewed the page again in a year's time it still wouldn't count it as an extra view.

It's important that Google get this number to reflect (as far as possible) the actual number of people who've actually seen the page as they are selling advertising based on the page views. The more page views the higher rates they can charge. Anyone running a site that has advertising has the same problem to solve and a lot of time and effort goes into getting these numbers as accurate as possible.

They won't publicise the exact algorithms they use as this will allow people to game the system and artificially inflate the viewing figures.

I would also suspect that, like on Stack Exchange, the viewing figure as displayed is cached. So when you view the page for the first time even though it registered the new view the figure on the page won't update for a few minutes (or even hours).