In the biblical Noah's Ark narrative, the Flood waters prevailed on Earth for 150 days.
Genesis distinguishes the 150-day period from the 40 days and 40 nights of rain. The rain is described as falling for 40 days, while the waters continue to rise or prevail for 150 days before beginning to recede. This is why answers based only on the famous rain number can be misleading.
The narrative then records a series of waiting periods. The ark comes to rest, the mountain tops become visible, and Noah sends out the raven and doves to test whether the ground is dry. Eventually God tells Noah, his family, and the animals to leave the ark.
Different passages and translations create complex chronological discussions about the Flood's total duration, but the specific statement that the waters prevailed for 150 days is explicit in Genesis. It refers to the dominance of the waters, not simply to rainfall.