Genesis says the Flood waters prevailed over the earth for 150 days after Noah’s Ark was afloat. This figure is separate from the 40 days and 40 nights of rain described earlier in the narrative.
The biblical chronology first describes rain falling for forty days and nights. It then says the waters increased and prevailed for 150 days before God remembered Noah and caused a wind to pass over the earth. The waters gradually receded, and the Ark came to rest on the mountains of Ararat.
Because the story contains several time periods, its chronology is often compressed in popular retellings. Forty days refers to the period of rain, while 150 days describes the period during which the waters prevailed. Later details include the opening of the Ark’s window and the eventual departure of Noah, his family, and the animals.
The number is therefore not a replacement for the rain duration. It answers a different question about the floodwaters’ dominance. Keeping those two periods separate avoids one of the most common numerical mix-ups in the Noah narrative.