So God obeyed by building an ark (Gen 6:22). How long would this enormous vessel require to build?

We are told (Gen 5:32) that Noah was five hundred years old when he began to father Shem, Ham, and Japheth. There is a short excursus about the descent of evil upon the world and God’s resolution to destroy it (6:1-7), and then the narrative returns to Noah, where a new toledoth begins (6:9) and we are told again that Noah has fathered his three sons. There is no further narrative until we are told God instructed Noah to build the ark. This seems to imply that Noah started the ark around the time of, or shortly after, the birth of his first son. The next marker of time has it that Noah is 600 years old (7:6), so up to 100 years would have passed—that seems like enough time.