How often you need to replace a PVC bag depends on many factors of
storage (light, temp and humidity being the biggies). Eventually they
degrade and break down gaining a yellowish or brownish tint. Estimates
on time between changing PVC bags is Anywhere from 2-3 years up to a
decade depending on how goog your storage conditions are. Bottom line,
PVC based bags are far from the best storage medium.
Archive grade PET (DuPont Mylar© D, Melinex © 516 or equivalent)
polyester is considered by most in the archival and conservation
fields to be the most "safe" plastic and is the only type of
protective plastic that is recognized for preservation of collections
by The United States Library of Congress Preservation Office, which
has very stringent requirements for the "protective films" used for
their archival storage
I prefer archival quality mylar and/or mylites for bagging as they
don't breakdown/degrade like the PVC poly bags (i've never replaced a
mylite yet and never had a mylite "go yellow", whereas I have had
polybags start to go yellow before I replaced them)
As for boards, they work just as good *outside* the bag as they do
inside, with the benefit that you don't need to worry about replacing
them when they're outside the bag as they're then not in contact with
the books.
I have kept comics in mylar bags for over 25 years. And I have kept some
comics in PVC bags for the last ten years.
I have found the biggest factor which makes a difference is what you store
your bagged comics in.
My experience is: comics stored in PVC and acid-free boxes are way better
preserved than comics stored in mylar in regular cheap boxes. The boxes are
the main culprits of acid destruction by far. Acid free boxes with PVC bags
is by far the best option for cheap long-term storage.
PVC bags from the last 20 years or so are much better than the ones people
used over 20 years ago.
As far as backing boards are concerned, place them in-between your bagged
comics, not in the bag itself.
The musty smell is from having the comics in moist environments.
Me.