That is a tough question to answer because it depends on the form and age of the fresh yeast. That is, is it a large block of yeast purchased from a baker or foodservice company, or is it fresh yeast purchased in a small cube in the supermarket? Either way, you may not know the age of the yeast when you receive it, and fresh yeast starts to degrade fairly quickly once it is removed from its packaging and brought to room temperature. It has a high moisture content and a short shelf life (about 4-6 weeks, if unopened and maintained at around 35-40 degrees F). To be on the safe side, it is usually recommended that fresh yeast (refrigerated) be used within about two weeks from purchase.
While some yeast experts say that fresh yeast can be frozen for future use if special precautions are taken (wrapping tightly in foil and freezing at a constant temperature below 0 degrees F), it is generally not advisable to do so since ice crystals can form and rupture the yeast cells and reduce leavening power (by as much as 10%) and result in a soft, sticky dough that may be difficult to work with (because of the dough softening effects of glutathione leached or released from the ruptured yeast cells). At this point, it is also difficult to know what the actual leavening power of the yeast is to be able to measure out the required amount to use.