Let's say I am to watch a video where my friend is going to sing and the video is too long so I want to fast forward to the moment where he begings to sing, but I don't know what minute he sings in/at? What is the correct way to ask the question and what preposition do I use?
Both prepositions can be used, but the placement of the preposition plus the use of ordinals or numerals is crucial.
You can say:
He starts singing at the sixth minute
or you can say:
He starts singing six minutes in
That last one is a shortened form of, "He starts singing six minutes into the video."