Genesis says the floodwaters prevailed on the earth for 150 days. Genesis 7:24 states that the waters prevailed for 150 days before the account describes their gradual recession.
This figure is separate from the forty days and forty nights of rain mentioned earlier in the narrative. The rain and the period during which the waters remain dominant are not the same measurement. Genesis later describes the waters decreasing and the Ark coming to rest on the mountains of Ararat.
The chronology continues with additional dates: Noah’s window is opened after a period of waiting, birds are sent out, and the family eventually leaves the Ark when the earth is dry. Different readers and traditions arrange these statements into detailed timelines, but the 150-day statement itself is explicit.
The figure should also not be confused with the total duration of Noah’s stay aboard the Ark, which is longer in the Genesis chronology. The narrative distinguishes rainfall, prevailing waters, and the complete drying of the ground.