How long did it take Noah to build the ark?
It does not actually say how long it took Noah to build the Ark. But we infer from Genesis 6:3 that it took 120 years: "And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years" (Genesis 6:3).
Compare "By faith Noah, being warned of God of things not seen as yet, moved with fear, prepared an ark to the saving of his house[hold]; by the which he condemned the world, and became heir of the righteousness which is by faith" (Hebrews 11:7).
Noah followed Instructions
Noah was apparently the only believer alive immediately before the Flood. The Flood generation was condemned for unbelief. This unbelief showed itself in the extreme disregard for God which characterised the period: "And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually" (Genesis 6:5). Noah was the exception: "But Noah found grace in the eyes of the LORD" (verse 8). He believed God, and moved by reverence for God, not abject fear, he obeyed instructions and built the world's first boat. The plans were given to him by God. If we build a model Ark to the same specification, it will float- unlike the boat in the Babylonian Flood legend, said to be a perfect cube, which is totally unseaworthy.
Noah preached Righteousness
St Peter draws a parallel between God's saving of Noah by faith, and of Christ saving us by faith: "For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but made alive by the Spirit: by whom also he went and preached to the spirits in prison; who at one time were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, in which few, that is, eight souls were saved by water. The like figure to which even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ: who is gone into heaven, and is on the right hand of God; angels and authorities and powers being made subject to him" (1 Peter 3:1822).
The Patience of God
When did the Spirit of Christ preach to the souls now in "prison" in Hades? Was it between the Crucifixion and Resurrection of Christ? No: it was before the Flood, while the Ark was a-building. The watching world saw Noah building, and still did not believe. It thus appears that for all the while the Ark was under construction (120 years?) the Spirit of Christ preached through Noah "the preacher of righteousness" (2 Peter 2:5) to his disobedient contemporaries. The righteousness which Noah preached to his generation was the righteousness which is by faith (Hebrews 11:7); but the only ones who believed and shared his faith were the members of his immediate family: "eight souls" (1 Peter 3:20). God's patience waited 969 years until the death of the oldest man who ever lived, Methuselah (which means "when I die it happens"), before the Flood came and swept them all away. Noah didn't live as long as Methuselah; but it is reasonable to assume that the last 120 years of Methuselah's life during which God's Spirit did "strive with man" were precisely those years during which the ark was a-preparing.
Was not 120 years the new reduced human Lifespan?
The 120 years was not the "new" human lifespan, as some argue; it was the Flood generation's last period of grace before the disaster. Only people like Moses, Joshua, Caleb, Jacob/Israel, and Joseph seem to have managed as much as 110 or 120 years. As Moses wrote concerning those born after the Flood, "The days of our years are threescore and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off and we fly away" (Psalm 90:10).
What does the Ark represent to us?
It is often said that the Ark represents the Church; but the Ark is not the Type of the Church. Methuselah is the Type of the Church. His name means "when I die it happens". When he departed the Flood Judgment happened. When the Church "departs" (is raptured) then the Great Tribulation Judgment falls. The Ark is the Type of Christian Baptism: "the like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ" (1 Peter 3:21). In Christian Baptism, our souls are "saved by water" (1 Peter 3:20); because we are thereby baptised into Christ's death (Romans 6:3-4). His death was for our justification, by faith; and his perfect life was for our sanctification, by faith (Ephesians 2:8-9).
Like Noah, we too have found grace in the sight of the LORD.