A day is 64 Minutes? Last I checked, the clock advances by 1 minute game time (GT) for every second in real time (RT).
So, going from 8AM GT to 8PM GT takes 12 Minutes RT. I am not sure about the night time length, but 7 minutes sounds about right.
It should also be noted that you can have infinite daylight as long as you do not complete the mission Airdrop as that is when the day/night cycles begin and you can see a clock. Source: Played for several hours exploring before doing the Airdrop mission. Tested on PS4. Mcdude man (talk) 15:06, January 28, 2015 (UTC)