title_body: How does boil time directly affect ABV% & IBU?
upvoted_answer: First, it's important to note that the boil time is not the only thing that increases with each of those beers.  Dogfish Head continually hops during the boil, and the boil extracts the alpha acids from the hops, giving the beer bitterness, so a longer boil with more hops results in a wort that is more bitter. 
  
  Here is where the additional alcohol comes in:  To counteract the extra bitterness, the beers that have boiled longer have both additional malt added to balance the bitterness, and are given additional fermentation and conditioning time to allow the yeast to convert all of those malt sugars into alcohol.  In 120 Minute IPA's case, it spends an entire month in fermentation, resulting in an exceptionally high level of alcohol at the end.