The author answered the out-of-universe reasoning in a series of tweets in 2016.
Q. How long is a day in the Grey Lands? Also why is it 3 mins and 4 seconds?
PÓG: Nobody really knows. It feels like a day, but is it? 3 minutes and 4 seconds was a decision I made because I wanted it to be long
enough for tension to form, but not so long for tension ...