Though there are many speculations about this, the Bible does not specify how long it took Noah to build his ark.

However, in Genesis 6:1-3, we learn that God gave the wicked people of Noah’s day an additional one hundred and twenty years to repent. At some point during this period God instructed Noah to build the ark. At the end of the one hundred and twenty years the flood came upon the earth. We are not told the exact time required to build the ark; only that it was constructed “during” the period of grace granted by God. So it must be something less than 120 years - the number thought by a lot of people to be the right answer.