Adam Clarke on Genesis 7:4

For yet seven days - God spoke these words probably on the seventh or Sabbath day, and the days of the ensuing week were employed in entering the ark, in embarking the mighty troop, for whose reception ample provision had been already made.

Forty days - This period became afterwards sacred, and was considered a proper space for humiliation. Moses fasted forty days, Deu 9:9, Deu 9:11; so did Elijah, 1Kgs 19:8; so did our Lord, Mat 4:2. Forty days' respite were given to the Ninevites that they might repent, Jon 3:4; and thrice forty (one hundred and twenty) years were given to the old world for the same gracious purpose, Gen 6:3. The forty days of Lent, in commemoration of our Lord's fasting, have a reference to the same thing; as each of these seems to be deduced from this primitive judgment.