The Bible does not give an exact account of how long Noah preached before the flood, but it states that it took Noah 120 years to build the ark. The Bible also states that Noah was 600 years old when the flood covered the earth.
Genesis 5:32 -10:1 states that Noah became the father of three sons at the age of 500. Noah's sons were grown and had wives of their own at the time of the flood; many of these years are unaccounted for in the Bible. This is also the time that God repented creating man and instructed Noah to build an ark to protect his family and the animals. The flood came 100 years later, so it may be assumed that Noah preached 100 years before the flood.