Hop flavor and aroma longevity is a mystery that I'm trying to unlock. Somehow I became a hop head last year while trying to free up some room in my freezer. It may not be much, but here's what I've uncovered.
Normally I ferment and primary for 4 weeks. Then I rack to a keg. I usually dry hop right away, but I've also waited for several months. Aging, or any delay in adding dry hops, doesn't seem to matter as long as they sit in the beer week or ten days. At that point the beer is plenty hoppy. Kegging offers dry hopping advantages that you obviously don't have with bottling.
Once the dry hops are removed the flavor and aroma drops off quickly. But it levels off after about two weeks and the drop beyond that is not so apparent. I haven't tried it yet but I may dry hop a second time if a keg loses too much hoppiness. Who knows DDH or double dry hopping may catch on
I've cut back late additions and I dry hop nearly everything now. It just seems like a better way to go, for me. I usually dry hop with Simcoe, Centennial, Citra or Amarillo -- the latter two being my favorite combo. I've tried 1 to 4 ounces. Two ounces -- combined not each -- per 5 gallons is the sweet spot for me. I don't notice a big increase flavor and aroma above that. It's not worth the extra hops and the reduction in beer (from absorption).
I always use leaf hops in a nylon hop bag. Sediment is never a problem. Getting the bag out once the hops have absorbed beer is. Recently I left dry hops in until the keg was half empty. That worked out pretty well. I'm racking a batch this weekend. I'm going to try leaving dry hops in for as long as there is beer in the keg.
One trick I learned is to use dental floss to suspend the hop bag in the keg. When I used string the CO2 pressure would leak from where the string met the o-ring (in a corny). That doesn't happen with dental floss. So I can pressurize the keg with the hops suspended, hopefully just off the bottom. I also put a couple of hefty stainless steel bolts in the bag so it sinks.
I have a keg of pale ale that I brewed last August. I wasn't drinking it so I yanked it from my keezer. It's just been sitting. It's not a bad beer, but it's just sort of boring. Two weeks ago I decided to add dry hops to maybe salvage it. I went with Mt Hood this time. Hopefully it made a blah beer into something more interesting. Hops are where it's at. I just wish someone would plant more Amarillo.