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.