This is a subject that has long intrigued me.
For example, on the commercial side, the advice on the use and storage of yeast tends to be very conservative. See, for example, Tom Lehmann's PMQ Think Tank post at http://thinktank.pmq.com/viewtopic.php?p=50956#p50956
. See, also the Lallemand chart at the bottom of page 1 at http://www.lallemand.com/BakerYeastNA/eng/PDFs/LBU%20PDF%20FILES/1_6DRYYE.PDF
, and also the Lallemand article Working with Instant Yeast at http://www.lallemand.com/BakerYeastNA/eng/PDFs/LBU%20PDF%20FILES/2_9INST.PDF
. Since professionals can go through large amounts of yeast in the course of their pizza operations, and usually in fairly short order, I suspect that their yeast remains quite fresh and that they are able to abide by the advice given to them by the experts like Tom Lehmann and the yeast producers. And it makes good sense to follow that advice since they do not want to put their dough production at risk simply because they chose not to heed that advice. For them, yeast is too cheap to put their production at risk.
However, on the home side, the above advice is routinely ignored. We have many members who, in the interest of being economical, purchase their yeast, and especially dry yeast, in one- or two-pound bags. Whatever they do not need to use at any given time they either store in their freezers or in their refrigerator compartments. Some will remove a small amount from the freezer to the refrigerator compartment for near-term use and return the rest to their freezers. Tom Lehmann recommends that yeast that is to be stored for future use be put in a airtight sealed container. And before putting the yeast in the sealed container, he suggests that the bag be closed securely, as by closing the bag tightly and wrapping a rubber band around it. That is the approach that I use to store my dry yeast in my freezer. '
According to the Lallemand yeast people, and as noted in the second pdf document referenced above, IDY does lose some of its leavening power with age. As the article mentions, IDY activity may decline 10 to 15 percent over the first one to two years after production, then more slowly after that. And that apparently is for IDY still in its unopened package. Lallemand says that if an opened package is stored in a closed container in the freezer, it will remain stable for at least three months. For many of our members, that period is not months and can often be measured in years. Since aging of yeast under these circumstances no doubt causes even further loss of leavening power, one way of compensating for that loss is to use more of the yeast than called for by the dough recipe. In my case, I use the volume measurements for yeast as specified by the dough calculating tools. Having selected the conversion factors for yeast for the dough calculating tools, I know that those numbers are not exact, simply because I know that yeast changes in so many ways during aging and storage and under different environmental condition. I also know that members don't always use level measurements of yeast. They might use a "scant" or "rounded" measurement instead of a "level" one. So, its anyone's guess as to how much yeast is actually being used in any given case. Even weighing the yeast is not a guarantee of accuracy since the weight of dry yeast will change with age. And some digital scales, even good ones, do not always accurately measure out small amounts of yeast. I compensate for all of these factors by simply using a bit more yeast than the recipe or the dough calculating tools call for. This might alter the fermentation process in one direction or the other, but the difference might not be enough to worry about.
I think it is also telling that following the above methods I cannot recall having lost a single pizza because of a yeast failure. I have read of yeast problems by other members, and sometimes I am perplexed why they sustained losses when they seemed to have done everything correctly, but perhaps just as often it was due to improper use of the yeast, and especially ADY.