Personally I keep them for much longer.

for the most part I only need to use mine once a day, and have noticed
not ill effects from keeping it for 2 months....

I use mine twice a day so it's pretty much all gone by day 30.  I do
occasionally forget a dose (several times a month) so it does last more like
31-32 days.  I've cracked open an empty inhaler, and the medication inside
is well sealed until you push the lever to prime a dose.  That dose is then
exposed, but the remaining ones are snug in their tiny little bubbles.

--
Liz

I've asked a number of pharmacists about this issue as the problem is common.
"In general" medications kept in reasonable storage (cool and dry) will last
many times their expiration date. Mostly they will just become slowly less
effective, not dangerous or poisonous. If I were to make a guess I'd think
Advair would be quite safe and effective for 2-4, maybe even 6 months. As the
person who took one apart mentioned the packets of medicine are individually
foil sealed until the unit is cocked exposing only one dose. I don't really see
how merely opening the package creates more exposure for the medicine.

An exception would be antibiotics-mostly these are prescribed to be ALL taken
over a relatively short period of time. Since antibiotic resistant bacteria are
on the increase and not a good thing to encourage in any form taking less
effective antibiotics is undesirable and could lead to personal super-infections
or other problems.

Fritz, RCP