Genesis 6:3 And the LORD said, "My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years."
This does not appear to relate to man’s lifespan because the generations which followed Noah through to Abraham lived more than 120 years
Noah lived 950 years Genesis 9:29 So all the days of Noah were nine hundred and fifty years; and he died.
Noah’s son Shem lived 600 years Genesis 11:10 This is the genealogy of Shem: Shem was one hundred years old, and begot Arphaxad two years after the flood. 11 After he begot Arphaxad, Shem lived five hundred years, and begot sons and daughters.
Shem’s son Arphaxad lived 438 years Genesis 11:12
Arphaxad’s son Salah lived 433 years Genesis 11:14
Salah’s son Eber lived 464 years Genesis 11:16
Eber’s son Peleg lived 239 years Genesis 11:18
Peleg’s son Reu lived 239 years Genesis 11:20
Reu’s son Serug lived 230 years Genesis 11:22
Serug’s son Nahor lived 148 years Genesis 11:24
Nahor’s son Terah lived 205 years Genesis 11:32
Terah’s son Abram lived 175 years Genesis 25:7
Thus, we see that for some 11 generations after God said “his days shall be one hundred and twenty years” people lived much longer. Thus, we must conclude that God is setting a time limit for Noah’s generation after which he would bring the flood.
His days shall be a hundred and twenty years. “His days” are the days of man, not the individual, but the race, with whom the Lord still strives. Hence, they refer to the duration, not of the life of an individual, but of the existence of the race. From this we learn that the narrative here reverts to a point of time before the birth of Shem, Ham, and Japheth, recorded in the close of the preceding passage as there were only a hundred years from their birth to the deluge. This is according to the now well-known method of Scripture, when it has two lines of events to carry on. The former narrative refers to the godly portion of mankind; this to the ungodly remnant.
Not forever will the Lord strive with man; but his longsuffering will still continue for one hundred and twenty years. Meanwhile he does not leave himself or his clemency without a witness. He sent Noah with the message of warning, who preached by his voice, by his walking with God, and also by his long labor and perseverance in the building of the ark. The doomed race, however, filled up the measure of their iniquity, and when the set number of years was accomplished, the overwhelming flood came.
I hope you find this helpful.