In Genesis, Noah waited forty days after the mountain tops appeared before opening the Ark’s window.
Genesis 8:5 says that the waters continued to decrease until the tops of the mountains became visible. Verse 6 then states that after forty more days Noah opened the window of the Ark. This is a later forty-day interval, distinct from the forty days and nights of rain at the beginning of the Flood.
The chronology can be difficult because Genesis records several separate time markers: the rain lasts forty days and nights, the waters prevail for 150 days, the Ark rests on the mountains, the mountain tops appear, and Noah waits another forty days before opening the window.
This question therefore requires careful reading of the sequence rather than recalling the most famous number in the story. The text’s “forty days” here describes Noah’s wait after the mountains became visible.