This is my favorite explination. Attached.
I did not understand why when I added hops at 30 min the hop flavor was still so strong. Beersmith told me that the IBU would be half but the flavor was twice as strong.
According to the attached chart the flavor peaks at about 20min, blue line. So instead of reducing the hop flavor by 50% I was increasing it by 35%.
The down side is that my beer flavor is not stable. After 3 months the hop flavor is almost gone. I think, and I need help here, it's because the alpha-acids are only about 50% strength at 30min, red line, so it doesn't release the volumn needed stabilize the flavor as expected.
Short answer: It takes 60 minutes to get all the alpha-acids out of the hops, I think.