According to Genesis, rain fell for 40 days while Noah was aboard the Ark during the Flood.
Genesis 7:12 gives the figure after Noah, his family, and the animals enter the vessel. The same chapter says that the waters continued to prevail on the earth for much longer, so the 40 days of rain should not be confused with the entire duration of the Flood.
The biblical narrative describes a further period during which the waters rose and covered the mountains. Genesis later records that the Ark came to rest and that the waters gradually receded. This produces several different time spans in the story, including 40 days of rain and 150 days during which the waters prevailed.
The number 40 is also a recurring biblical period associated with testing or transformation. It appears elsewhere in stories involving Moses, the Israelites, and Jesus, although those episodes are separate from Noah’s Flood.