How Long Do Bittering Hops Need to Be in Boil?

So I put my bittering addition it at 60 min. At what point are all the alpha acids washed away from hop cones? Is there a point where I could pull out the hop cones, but the alpha acids would still be left behind?

Seems like if the alpha acids are in the wort after say 15 min, boiling hop cones just increases tannin extraction...

So the thing with alpha acids is that they are fairly volatile but they convert in a rolling boil to iso-alpha acids and they are what give you a lot of bitterness. It basically comes down to what % of conversion you want, but there are diminishing returns. If I remember correctly, at 60min, you get about 30%, at 90min you can get 33% conversion, after that you start getting a strange vegetable flavor from boiling them.

There is currently a school of thought about hop blasting or something where all of the hops are added in the last 15min. There are a lot of hops added to get a large amount of iso-alpha acids not from the long boil, but one from the volume of hops. At this point I make traditional German beer which doesn't lend itself to this style of brewing but a lot of people on this forum do it.

Great beer and explanation of late hopping http://www.mrmalty.com/late_hopping.php

The only charts that I have seen go upto a 90 min boil. There is some decreased but continued utlization through out the 90 minutes. The highest utlization comes throughout the 60 minute boil.