In the Book of Genesis, the Flood waters prevailed for 150 days after Noah entered the Ark.
This figure comes from Genesis 7:24, which states that the waters prevailed upon the earth for 150 days. It is distinct from the forty days and forty nights of rain described earlier in the account. The two numbers refer to different stages of the Flood narrative.
Genesis then describes the waters gradually receding. The Ark came to rest on the mountains of Ararat, and Noah later sent out birds to test whether the land had become visible. The story therefore presents a lengthy period of rising, prevailing, and receding waters rather than a brief storm.
Readers sometimes combine the 40-day rainfall with the 150-day period of prevailing waters. They are not interchangeable. The 150-day figure concerns the continuing dominance of the Flood waters, while the forty-day figure concerns the rain itself.