The alpha acids will drop over time - a longish time. Aroma hops will lose some of their aroma too. Keeping them airtight and refrigerated or frozen will increase their storage life. As to how long... I don't know from personal experience, I brew with them pretty quickly usually. They should be OK in the freezer for a number of months. You might need to use just a tiny bit more (say 5-10%) once they've aged. It all depends on how well they've been stored.
Just one point of interest is that it is apparently not good to store them in the freezer as the low temperatures rupture the cell walls and stuff up the aromas .... good thread on Grumpy's about this Hop thread @ Grumpy's.
I just keep mine in a airtight container in the fridge.
I remember that message - it was from John Ross at Hopco. He'd been to a brewery where they'd successfully used aroma hops after 6 years - aroma was fine - alpha acids had dropped away to blazes. From memory I think they'd been in cold storage but not at freezing. I think I remember him saying that they recommend storage around 0 degrees - freezing is fine. If you want to find the original message, try a search on the craftbrewing list.
For those that have a copy of promash, you can input the AAU% and set the temp of storage and get a time line of AAU% degredation. This is done in the area labelled "Hop Time".I inputed SAAZ at 3.5 % and wound it down to -7oC and it lost 1%AAU over 12 months according to the programme
Couldn't find John's response Reg but I did find this from you (seems to agree with most of the other opinions on Craftbrewer):
John Ross from Hopco can give a definitive answer on this question.
However my understanding is that Hopco recommend storage at 0-2
degrees, and hops stored in a freezer will have a longer shelf life
than those kept in the fridge. There's no need to purge repacked hops
if you freeze them.
John has previously mentioned that brewers have kept aroma hops for up
to 6 years in normal cold storage and found the hops still do the job
required. During this time the alpha will have fallen to very low
levels but this does not matter with the aroma hops which are used
late in the boil.
He also argues that different hops have varying shelf lives. Pride of
Ringwood (considered a poor shelf life hop) normally lose 15 - 20% of
their brewing value in the first year when stored at 1 - 2 degrees C.
i.e the original alpha will fall from say 10% to around 8.0 - 8.5%.
Similar degradation will occur over the next couple of years. Hops
kept in a freezer should lose very little alpha.
Hopco have created an interesting spreadsheet outlining varieties and
characteristics (eg. alpha acid, storage stability). I don't think
John would object to adding this to the files section on the website.
I store my hops in the freezer, double bagged. If I had CO2 or a vac.
pack, I'd purge or suck the air out, but that's just me. It's not
necessary with freezer storage - particularly given that we're
probably likely to not be storing hops for up to 6 years+ (at least
I'm not - any hop in my freezer pretty soon makes a trip to the
boiler). Maybe it becomes more of an issue if you're intending to
make a bulk purchase - eg. a 5kg batch of pellets??