According to Genesis, the Flood waters prevailed over the earth for 150 days before they began to subside.
Genesis 7:24 says the waters prevailed for 150 days. Genesis 8 then begins the turning point, stating that God remembered Noah and caused a wind to pass over the earth, after which the waters began to decrease. This figure describes the period of dominant high water, not simply the duration of rainfall.
The distinction is important because the same narrative gives other time spans. Rain falls for forty days and forty nights, the ark rests on the mountains of Ararat in the seventh month, and Noah remains in the ark until the ground is dry. These references describe different stages rather than competing answers to one question.
Some readers calculate the Flood chronology differently because Genesis includes month and day references under an ancient calendar system. The explicit statement about 150 days remains the clearest answer for the period during which the waters prevailed.