Most Christians are under the assumption that it took Noah 120 years to build the ark, because God said “My spirit will not remain in humankind indefinitely, since they are mortal. They will remain for 120 more years” (Genesis 6:3). Since this verse appears prior to God’s command to Noah to build the ark, it is reasoned that it must have taken Noah 120 years to finish the project. A careful reading of the text, however, will demonstrate that the ark was built in 80 years or less.
Genesis 5:32 says Noah became the father of Shem, Ham, and Japheth after he was 500 years old. If it took Noah 120 years to build the ark, the flood would have come when Noah was 620 years old. And yet Genesis 8:13 says Noah was 601 when the flood waters dried up. Given that the flood lasted about a year, Noah would have been 600 when he “set sail.” (see also Genesis 9:28) At best, that leaves 100 years to build the ark.
We can be more specific, however. According to Genesis 11:10, Shem was 100 years old when he fathered Arphaxad two years after the flood. That means Shem was 98 when the flood ended, and thus 97 when the flood began. If Noah was 600 when the flood began, then Noah must have fathered Shem when he was 503. We know from Genesis 6:18 that God did not instruct Noah to build the ark until after his sons were born, and after they had wives of their own. If we assume Noah fathered his three sons in three consecutive years, his last son would be born when Noah was 505 years old. And if we assume that his youngest son married at the age of 15, Noah would be no younger than 520 years old when God instructed him to build the ark. If he was 600 when the flood came, then the building of the ark could not have taken more than 80 years.
What, then, is the meaning of Genesis 6:3? I think we are right to understand it to refer to the amount of time before which God would destroy the Earth. What we are wrong to assume, however, is that God instructed Noah to build the ark immediately after making this decision. There is nothing in the text to warrant this conclusion, as there are no temporal indicators suggesting that God revealed Himself to Noah immediately afterward. We are told of God’s decision to destroy the Earth in 120 years in a separate narrative describing men’s wickedness. That narrative ends at 6:8, and a new narrative about Noah begins at 6:9. It is here that we read of God’s interaction with Noah, and His instructions to him to build the ark. Given what we know from other passages about the length of time it took Noah to build the ark, there must be at least a 40 year gap between Genesis 6:3 and 6:13-21.