Bill Mares: By asking, you have done more than most.  My compliments!
Apistan strips are a CONTACT pesticide, meaning that as bees touch it, some of
the active chemical, fluvalinate, rubs off onto the bee.  Hence, the longer a
strip is hanging in a hive, or a hive with a "gangbuster" population, the
shorter the effective life of the strip.  In estimating the "average"
environment of colonies, the manufacturer has concluded that the strip life in
use is 42-56 days (6-8 weeks).  NOTE: IN USE.  Unfortunately, many beekeepers
have used them, removed them, washed them clean, and put them in their freezer
until next year, and then  proclaimed loudly: "I used Apistan strips, and they
DON'T WORK, because the mites killed my bees".
If the strips are kept away from heat, and sealed up in the DARK, I don't
think they have been in use long enough to measure length of life before
deterioration.
Dr. Bill Wilson of Tucson Lab reported strips in excess of 5 years old being
100% active upon use after being kept sealed and in the dark for those five
years.
To save money, I buy in quantities of 1000 at a time.  By my accident, I have
misplaced a box of 100 in my freezer, but they were excellent two years later
when I found them.
BUT IF YOU USE THE AT ALL IN A HIVE, do not try to save them to use again!
4 Apistan strips cost about $6.  A replacement 3lb. package of bees costs
about
$35, and you have lost a year.  In my book, that is penny wise and pound
foolish!
Apistan works about 99% of the time, in 99% of areas, and with 99% of the
users.
No one knows about that other 1%.  My bees are always free of mites!
George Imirie