How long does someone have to watch a Facebook story in order to be noted as "seen"?

I have read somewhere that if someone watches the story for less than 3 seconds, it doesn't count as "seen" and the name of that person doesn't appear on the list of people who have seen it. Is it true?
Also, how is it possible that, for example, if my story consists of 5 photos, someone only sees the third, and not the first two as well? It would make sense if someone saw the first, and then decided he doesn't want to see more and closes the story, but how can someone only see a photo in the middle?

?2019-01-04T20:37:09Z

Hello, There is no amount of time required to be marked as seen by a user. it works instantly. Just like the seen option on other platforms such as the Status Feature on Whatsapp.

Aqualina2019-01-03T11:39:44Z

That is wrong. A Facebook story is marked as seen in the exact moment you see it.

Daniel2019-01-03T11:38:49Z

Not true the Seen is Recorded Instantly once you Start Watching It

Qidong Jiangsu2019-01-03T11:38:47Z

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