In the Genesis Noah's Ark narrative, the floodwaters prevailed for 150 days.
Genesis 7:24 says that the waters prevailed upon the earth for 150 days. This is not the same as the forty days and forty nights of rain mentioned earlier. The rain describes the period of intense precipitation, while the 150-day figure describes how long the waters remained dominant in the account.
Genesis 8 then says that God remembered Noah, the waters began to subside, and the ark came to rest on the mountains of Ararat. The chapter continues with a detailed sequence of waiting periods before Noah and his family leave the vessel.
Some translations and chronological readings discuss the flood timeline in slightly different ways, especially when counting months and days. The 150-day statement itself is explicit, making it the standard answer to this question.