According to Genesis, the Flood waters prevailed for 150 days.
Genesis 7:24 states that the waters prevailed upon the earth for 150 days. This figure follows the account of the rain and rising waters and describes the period during which the Flood remained dominant, rather than merely the time rain was falling.
That distinction explains why 150 days is different from the famous 40 days and 40 nights of rain. Genesis then describes the waters gradually receding, the Ark coming to rest, and Noah testing conditions with birds before leaving. The complete chronology is therefore longer than either 40 or 150 days alone.