In the Genesis flood narrative, Noah waited 40 days before opening the Ark’s window. Genesis 8:6 says that after the waters had receded, Noah opened the window he had made in the Ark.
This 40-day interval is distinct from the 40 days and nights of rain at the beginning of the story. It occurs later, during the process of determining whether the land had become dry. Noah then sent out the raven and later the dove, using the birds to gather information about conditions outside.
The timeline in Genesis includes several separate periods, which can be easy to merge. The rain lasts 40 days and nights; the waters prevail for 150 days; and Noah’s later actions involve waiting periods of seven days. Genesis 8:6’s 40 days specifically marks the point when Noah opens the window, not the end of the entire flood account or the moment he leaves the Ark.