Length of Days Question

Then the Lord said, “My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years.” Genesis 6:4

I've read this passage several times before but only recently read the Geneva Bible commentary that defined the 120 years as the amount of time before God's judgement with the flood.

For some reason, [and I'm not sure where I got this idea] I always thought the passage referred to an overall shortening of the average length of lifespan.

The commentary references 1 Peter 3:20 which does not mention 120 years, but does describe waiting for judgement while the ark was being built.

Which interpretation have you understood it to mean? Where did the idea of shortened lifespan come from?

I posed these same questions in a bible reading group and discovered I'm not the only one who was challenged with a different interpretation. If you have any additional resources or references to this passage, please share them in the comments.