In Genesis, Noah waited forty days before sending out the raven from the Ark.
Genesis 8:6 says that after forty days Noah opened the window he had made in the ark and sent out a raven. The raven flew back and forth until the waters dried from the earth. Noah then sent a dove to check whether the floodwaters had receded from the ground.
The number is easy to confuse with other time periods in the same narrative. Genesis also describes seven-day intervals involving the dove, and it gives additional dates for the beginning and recession of the Flood. “Forty days” is specifically the interval stated before Noah first sends the raven.