Our ways of keeping time differ from Gods. One thing to keep in mind is that our calendar accounts for leap day every four years. 120 years for God is different than 120 years on our time. In 2 peter 3:8 & Psalm 90:4 the Bible contrasts 1000 years to 1 day. That doesn't necessarily mean that 1000 years for us is only 1 day to God, it just reinforces the idea that God has his own time.
Could it be that God's spirit will be with us for 120 years enabling us to call on him feeling his presence and that at the end of that time we will either understand and accept God without question or reject all that we have learned through our lifetime thus losing his spirit. For 120 years God is giving us the opportunity to become Godly.