The floodwaters prevailed for 150 days in the Noah's Ark story.
Genesis 7:24 says that the waters prevailed upon the earth for 150 days. This is a different measurement from the forty days and forty nights during which rain fell, making the two numbers easy to mix up.
The narrative then describes the waters gradually receding. Genesis 8 records the ark resting on the mountains of Ararat and later gives dates for the appearance of mountain tops and the drying of the ground.
The flood chronology uses dated months and days, creating a sequence that extends well beyond the initial rainfall. Thus, 150 days describes the period in which the waters remained dominant, not the complete time Noah spent inside the ark.