Genesis 6:3
ESV - 3 Then the Lord said, "My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
When God saw the evil of men (i.e "These were the fallen angels and the sons and daughters of the fallen angels") He gave them 120 years to repent. During this time Noah spoke to them and told them of the flood that was to come. God stopped his people from further child birth and when all the sons and daughters had died, Noah only spent 5 years building the ark. This is from the book of Jasher (an Apochrypal book) and mentioned twice in the Holy Bible.