In the Genesis flood account, the floodwaters prevailed on the earth for 150 days.
Genesis 7:24 gives this figure after describing the rising waters, while Genesis 8:3 says that the waters receded after the 150 days. This period is separate from the 40 days and 40 nights during which rain fell at the beginning of the flood.
That distinction explains why different numbers appear in summaries of the story. Forty describes the rainfall or rising-water phase in the narrative, whereas 150 describes the period during which the waters remained dominant before receding.
Genesis continues with dated milestones after this period, including the ark resting on the mountains of Ararat and the later drying of the ground. The text’s sequence is therefore longer and more detailed than the familiar shorthand that simply says the flood lasted 40 days.