I have been baking bread weekly for at least 15 years, and my experience
with the useful life of yeast is very different from herb lady's.
I have been using SAF Instant for a number of years, and before that I used
Red Star Instant Active (both packaged in 500 gm/17.6 oz bags). I all that
time, I have never had to discard a loaf of bread because of weak yeast
performance, nor have I discarded the yeast because it was past its "best
if used by" date. Just a few weeks ago, I finished the last teaspoon (the
amount I use for a large bread machine loaf loaf) of SAF Instant with an
expiration date of April 1999. I began using it after its expiration date,
kept it in the refrigerator (not the freezer), used it for over 2 years,
and never had any indication of weakening of the yeast before the bag was
empty.
The bottom line of my experience is that good yeast which is kept in the
refrigerator, and handled carefully (kept open only long enough to spoon
out the yeast, and then tightly closed with a rubber band in its original
500 gm foil bag), will last a very long time. In fact, I have no idea how
long it may last because I've never had to discard any!