The Bible does not specifically say how long it took Noah to build the ark. When Noah is first mentioned in Genesis 5:32, he is 500 years old. When Noah enters the ark, he is 600 years old. The time it took to build the ark would depend on how much time had passed between Genesis 5:32 and the time that God commanded Noah to build the ark (Genesis 6:14-21). At the absolute most, it took 100 years. Read more: http://www.gotquestions.org/Noahs-ark-questions.html#ixzz2UQZApeNQ
It took Noah around 120 years to build the ark. God pronounced the coming judgment of the flood 120 years before it happened. It is however argued that God gave the warning 20 years before giving Noah instruction to actually start building the ark making it 100 years to complete the project.
That's like aking how long did it take Huck Finn to build his raft. Both the story of Noah and Huck Finn are fictions lizard accounts told to teach a deeper truth. At least that's how adults look at it.