The exact time when Noah built the ark is unknown. However, a rough estimate is that it took less than 100 years to build. Noah set out to the ark with his wife and three sons. He had just had his last son while his eldest was old enough to be married. God instructed Noah to build an ark and get on it, in preparation for the massive flood.
This saved civilization as everything else which did not make it to the ark was destroyed. The flood lasted for approximately 150 years, which was a short time for Noah. Finally, ten days after the boat hit shore, it had reached Mount Ararat in the seventh month.