How Much Longer For The Gene For Blue Eyes?
Never.

Even if the country arrives at the unlikely day when there is no one alive in the United States with the blue eye phenotype, there are still plenty of people with the unexpressed recessive gene, and if two of them get together, those genes can combine to produce another blue-eyed child. If this happens twice, with one boy and one girl, and they happen to meet when they are in college and say "Hey cool, you have the same inexplicably rare kind of eyes as me. I think I want to marry you." then any children they have will also be blue eyed.

So even if the day came when you couldn't find a soul in the world with blue eyes, you will still never know if you've seen the end of them.