The reason dry yeast is so popular is that it is easier to store, and is less persnickety about it than fresh yeast.
Treat dry yeast right
and it can last for a couple of years or more.
Fresh yeast is highly perishable, and it should be frozen if you're not going to use it within a couple of days.
If you're lucky, you can get significantly more time from fresh yeast by freezing it within just a day or two of getting it home.
The thing is, yeast is a living thing
and you just can't know what experiences it has had before arriving in your kitchen.
As such, it's impossible to predict just how much extra (past the expiration date) time you are going to get from freezing it.
A month?
Probably.
Two months?
I wouldn't count on it, but maybe.
6 months?
Highly unlikely, but stranger things have happened.
If you do freeze fresh yeast, wrap it very well.
Moisture and air are your enemies.
Do put it in the refrigerator at least 12 hours prior to use, then let it spend the last hour on the counter.
As a last step before putting together your dough, proof it.
Proof the yeast by mixing it with the warm (100Fish) water called for in the recipe and, if applicable, the sugar.
If there is no sugar in the recipe, give it a 1/2 tsp of flour (per loaf).
Within 5-10 minutes it should be quite bubbly and growing.
If it doesn't look like it's doing much, throw it away.
You might as well throw away all of the yeast you have from that batch.
Go to the store and get more yeast before proceeding with the recipe.
Properly stored, yeast usually dies at least somewhat gradually.
You may see a slight progressive decrease in the vigor of your yeast as it gets older.
Personally, once I see that I'll use it that time, but I'll get more for next time.
Anymore, fresh yeast is hardly seen except in professional bakeries.
It's easy to see why.
If you're going to make multiple loaves in a week, then maybe fresh yeast is worth it.
Now that I have found 2 pound packages of Fleischmann's instant dry yeast
(expiration almost 2 years out) at Sam's Club for $6, I think my days of messing with fresh yeast are over.
(BTW 2lbs of dry yeast = about 130 loaves) EDIT:
Interestingly, Red Star disagrees and doesn't recommend freezing fresh yeast.
This goes against my experience and the cynic in me wonders of their recommendation has more to do with selling yeast than anything else.
cooking.stackexchange.com/users/20183/Jolenealaska
I found this useful:
http://www.wessexmill.co.uk/recipe/freezeyeast.html As soon as I get it home
I crumble 12g [of the fresh yeast] into approx 30g (2 dessert spoons) of a ordinary bread flour and mix it up so that I have a dry crumbly mix, and put into a small plastic bag.
I do this to approx 70% of the yeast
so I have about 50 small bags i.e 1.5kg bag of flour.
I then put all the small bags into a big bag and put them in the freezer.
Mixed in with the flour and frozen
I find the yeast keeps for a couple of months
, it may keep longer, I don't know, I've usually run out by then and started again.
Good Luck
cooking.stackexchange.com/users/30944/MARSHA
Being a Professional Baker Fresh yeast cannot be frozen I have tried it is dead on defrosting and cannot be ressurected with sugar.
Neither do I like dried yeast
the bread dough is too heavy and takes too long to rise it spoils the taste of the loaf
cooking.stackexchange.com/users/41833/user41833
I disagree with eschewing fresh yeast.
I am trying to make bread like I used to get as a kid in Northern NJ
and it is pretty much impossible to get the dried yeast to provide the necessary lift.
The recipe I was trying to use shows the cook Dannielle Forrester on "Baking With Julia" using Fresh yeast.
I have been using starter added to the water with yeast and proofing it then using it was the water eliminating the adding fresh yeast part.
She mixes up the water and flour first then adds the fresh yeast and after kneading it adds the salt.
I'm getting the right crust but the inside is just not lifting and making the big holes as it should.
So I'm on the definitely use fresh yeast if it is called for side of the question.
I did a lot of searching online and calling yesterday and I am going to have to drive a couple hours to get it from a baker who is willing to sell me some of what he uses but its not on the shelf anywhere I could find in NW Oregon.
Pretty much everyone I spoke to had no idea what it was except for this one guy.
I found this page in my attempt to fond out how long I can freeze it as that will dictate how much I buy.
Maybe 4 months is the conclusion I see
so I'll probably only be getting a few ounces.
Thanks all
cooking.stackexchange.com/users/29424/Chris
Anyone who follows grandmas recipes knows that fresh yeast rises better and definitely makes your product taste different and better.
Coming from my roots in New York, any baker worth his weight in gold uses fresh yeast in their products.
As far as whether you can freeze fresh yeast it is a crap shoot.
I have had it rise fine and other times
it's has not and you have ruined all your ingredients.
My recommendation is always to buy it fresh at the store.
Some Wal-mart stores carry it by their biscuits doughs or eggs.
Problem is they order to much and is usually out dated.
That's why most stores stop carring it.
They are loosing money.
If they bought smaller quantities more often they would sell more
cooking.stackexchange.com/users/45915/Danny
I've heard that I can actually buy a large box of fresh yeast cubes, freeze it, and take one out of the freezer twelve hours before using it, to let it slowly get to room temperature again.  How long can I freeze the yeast for? Will it slowly degrade, or will it just be suddenly unusable after a certain length of time?
How long can fresh yeast be frozen?
cooking.stackexchange.com
<freezing><yeast>
cooking.stackexchange.com/questions/40134
cooking.stackexchange.com/users/6143/takrl
You may be able to freeze yeast soon after buying and defrost it successfully, but this is unreliable and not recommended. There are very different opinions about freezing fresh yeast. While amateur bakers appear to belive it is ok, professional bakers tend to disagree. Others believe that there is no guarantee the yeast will be usable after freezing.
The life of fresh yeast can be prolonged by freezing. Professional bakers tend to believe fresh yeast should not be frozen. Freezing fresh yeast is hit and miss: there is no guarantee it will live.