In the Genesis flood story, the floodwaters prevailed over the earth for 150 days.
Genesis 7:24 says that the waters prevailed upon the earth for 150 days. This figure follows the account of the 40 days and 40 nights of rain, so it is not a replacement for the rainfall figure. It refers to the period during which the waters remained dominant and the land was submerged.
Readers often combine the two numbers because both occur close together in the narrative. Genesis 8 then describes the waters receding and the ark coming to rest on the mountains of Ararat. The text’s chronology is presented through dated stages, including the month and day when the ark rested and when the ground became dry.