Christians usually teach that Noah spent 120 years building the ark before the Flood came, but the biblical basis for that teaching is weak. The only mention of "120 years" in relation to Noah occurs in Genesis 6:3, where the exact meaning is ambiguous. Here's how the King James Version puts it:
"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, KJV
The traditional interpretation is that Noah had 120 years of warning before the Flood, but commentators are divided, with some of them asserting that God was not initiating a 120-year doomsday clock but stating his decision to limit human lifespans to no more than 120 years.
To me, the interpretation that Genesis 6:3 refers to human lifespans is more convincing. I can't argue from a translation or theological standpoint, but having studied and practiced writing, I can argue that the author(s) of Genesis intended for readers to read that promise as referring to how long people live. Here are my observations.
The primary argument against Genesis 6:3 referring to human lifespans is that most of the lifespans Genesis records after that verse are longer than 120 years. However, Genesis ends with the first patriarch to live less than 120 years:
"So Joseph died, being an hundred and ten years old: and they embalmed him, and he was put in a coffin in Egypt." —Genesis 50:26, KJV
Genesis 6:3 and 50:26 form a Chekhov's gun in the overarching narrative of Genesis, a promise made in act one that has to be fulfilled by act three or becomes superfluous. The very last chapter Genesis fulfills the promise made in chapter six: human lifespans no longer exceed 120 years, not even for God's favored people.
In fact, one hundred twenty years still seems to be the upper limit of human longevity, as only one person is documented as living beyond it (and not without skepticism). It's plausible that ancient observers noticed a similar limit on human lifespans, causing biblical writers to include Genesis 6:3 as an etiological explanation for why that limit exists.
The contrast between Joseph's age and the antediluvian patriarchs who lived for centuries isn't spurious. A few chapters earlier, Genesis prepares the reader with some foreshadowing:
"And Jacob said unto Pharaoh, The days of the years of my pilgrimage are an hundred and thirty years: few and evil have the days of the years of my life been, and have not attained unto the days of the years of the life of my fathers in the days of their pilgrimage." —Genesis 47:9, KJV
That deliberate commentary and contrast, made by the last patriarch to live longer than 120 years, marks the transition between those who transcended the limit and those who, henceforward, won't.
The implicit theological point made by Genesis 6:3 and Joseph's premature death is, then, that God is in control of human longevity.
As an aside, there is once piece of false evidence to mention. Jacob triply repeats the phrase "the days of the years", which is easy to connect with the similar "days...years" construct employed in Genesis 6:3:
"...yet his days shall be an hundred and twenty years." —Genesis 6:3, KJV
However, when Jacob's phrase "of the years" appears to be an insertion of the King James translators. The Hebrew only says "days". Similarly, the King James translation of Genesis 5 repeatedly uses a "days...years" equivalence to describe the lifespans of the antediluvian patriarchs, which seemingly establishes an idiomatic precedent for reading Genesis 6:3 as referring to lifespans. Unfortunately, that idiom appears to be a part of King James-era English, since the original Hebrew in Genesis 5 only mentions "years". I wonder if early English translators of Genesis interpreted Genesis 6:3 as referring to lifespans more easily than Christians do today.
As for arguments against the traditional interpretation, the first is that an interval of 120 years before the Flood is arbitrary. God could just as well have granted 90 or 150 or 200 years without any loss of meaning. A 120-year reprieve is a reprieve whose length has no narrative significance. Contrast that with 120-year lifespans, which not only has a narrative touchpoint at the end of Genesis but can also be compared with observed human lifespans.
Another aspect that makes the interpretation of a 120-year reprieve suspect is its usefulness in today's Christian rhetoric. Rather than needing to demonstrate, as Genesis's original author(s) did, that God controls human lifespan, today's preachers need a way to support the imminence of the end of the world while also excusing its perpetual delay. Noah having more than a century of forewarning before the Flood provides a useful narrative for that modern rhetorical need. A 120-year era of probation for the antediluvian world allows Christians to preach that judgment day is just around the corner but, as in the days of Noah, that corner may be the size of a lifetime.