Or HaChaim on Genesis 8:6/אור החיים על בראשית ח׳:ו׳

1 ויהי מקץ ארבעים יום. It was at the end of a period of forty days. How did Noach know when the time had come to safely open the window? Was he not scared the surrounding waters would flood the ark? Perhaps G'd had told him that the deluge would not last longer than twelve months and he had prepared a food supply to last for that period of time.