According to Genesis, the Flood waters prevailed over the Earth for 150 days.
Genesis 7:24 gives the 150-day period after describing the waters covering the high mountains. Genesis 8:3 repeats the figure while explaining that the waters began to recede after that interval. This number describes the period during which the waters remained dominant, not simply the duration of rainfall.
The distinction matters because Genesis also says that rain fell for 40 days and 40 nights. The two figures refer to different stages of the story: rainfall is the initial deluge, while 150 days covers the continuing presence and movement of the waters. The narrative then continues with the ark resting, mountain tops becoming visible, and the ground gradually drying.
Flood traditions outside Genesis use different measurements and sequences. Even within discussions of Genesis, readers sometimes combine the 40-day rainfall with the 150-day water period as though they were interchangeable. They are separate chronological details in the biblical text.