In the Genesis Ark story, rain fell for 40 days and 40 nights. This period is stated in Genesis 7:12, where the Flood narrative describes the waters covering the earth after Noah entered the Ark.
The 40-day rainfall should not be confused with the total time that Noah and the animals remained aboard. Genesis also says that the waters prevailed on the earth for 150 days, and the complete sequence—from entering the Ark to leaving it—lasted much longer than the rainfall itself.
The number 40 has recurring importance in biblical literature. Other examples include Moses spending 40 days on Mount Sinai and the Israelites’ 40 years in the wilderness. In the Flood account, however, the specific reference is to rain continuing for 40 days and 40 nights, not to the entire duration of the catastrophe.