According to Genesis, Noah sent out the raven 40 days after the mountain tops appeared. Genesis 8 describes the Flood waters gradually receding after the Ark came to rest on the mountains of Ararat.
The text says that, in the six hundredth year of Noah’s life, the waters continued to decrease. By the first day of the tenth month, the tops of the mountains became visible. After another 40 days, Noah opened the Ark’s window and released a raven.
The raven flew back and forth until the waters had dried from the earth. Noah later sent out a dove on several occasions to test whether the ground was dry. This 40-day interval is separate from the 40 days and 40 nights of rain mentioned earlier in the story, and it is also different from the longer period during which the Flood waters prevailed.