Genesis says the Flood waters prevailed over the mountains for 150 days. Genesis 7:24 states that the waters prevailed upon the Earth for 150 days before the account describes their gradual decrease.
This number is different from the 40 days of rain mentioned earlier in the Flood narrative. Genesis 7:12 says rain fell for 40 days and 40 nights, while Genesis separately describes the waters prevailing for 150 days. The two figures refer to different spans of the story and should not be substituted for one another.
Genesis then says that God remembered Noah and caused a wind to pass over the Earth, after which the waters began to abate. The Ark comes to rest in the seventh month, and the ground is eventually dry before Noah leaves. Chronological details in the Flood account are discussed by biblical scholars because passages can use different time markers, but the explicit duration in Genesis 7:24 is 150 days.