According to Genesis, the floodwaters prevailed for 150 days after Noah's Ark was afloat.
The figure appears after the account of the forty days and nights of rain. Genesis says the waters increased, lifted the Ark above the earth, and continued to prevail for 150 days. This wording describes the period during which the floodwaters remained dominant, rather than simply counting the days of rainfall.
That distinction creates a common mix-up. The rain lasts forty days and forty nights, while the waters' broader period of prevailing lasts 150 days. Later in the narrative, the Ark comes to rest on the mountains of Ararat, the waters recede, and the occupants eventually leave. The biblical chronology includes several separate time markers, so readers should not treat forty and 150 as competing answers to the same question.