According to Genesis, the floodwaters prevailed over the earth for 150 days. Genesis 7:24 gives this duration after describing the rising waters and the destruction of life outside the Ark.
This number is different from the 40 days and 40 nights of rain mentioned earlier in Genesis 7:12. The rain describes one phase of the event, while the 150 days describe the period during which the waters prevailed.
Genesis 8 then begins the recession of the waters. The Ark comes to rest on the mountains of Ararat, mountain tops become visible, and Noah later sends birds to determine whether the land is becoming habitable.
The flood chronology is sometimes summarized as simply “40 days,” but that shortens the narrative considerably. Genesis presents rain, prevailing waters, recession, drying ground, and the eventual departure from the Ark as separate stages.