The waters kept receding steadily from the earth, so that they had gone down by the end of the 150 days.
Referenced Verses
Gen 7:24 : 24 The waters prevailed over the earth for 150 days.
Gen 7:11 : 11 In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month– on that day all the fountains of the great deep burst open and the floodgates of the heavens were opened.