Most of the papers that I have read have stated that viability tends to decline rapidly after 120 days. At 120 days, the viable cell count found in most cultures will be roughly 1/4th of the viable cell count at the time that the yeast was harvested. My own experience with yeast management tends to agree with this information.
If one reads the fine print on a White Labs vial, it states that the culture is basically guaranteed for 4 months from the production date. That number was not pulled out of thin air. It agrees with what I have posted above. Now, is a White Labs vial bad after 120 days? No, it's just that the cell count is down to around 15 to 25 billion cells, which is an order of magnitude away from the cell count necessary to pitch a 5-gallon batch of wort, and White Labs has always positioned their cultures as "ready to pitch." I often claim that a yeast culture is kind of like a nuclear weapon in that one only needs to be close for it to work correctly. An order of magnitude is not close, which means that the value of making a starter increases with respect to the distance between the production date and the current date. I currently have a batch fermenting that was pitched with a starter that was inoculated with a vial of WLP022 (Jan/Feb seasonal). The batch started in less than 12 hours.