During my activities in Sweden I have seen at least three Hupfeld
Phonolas with skin-covered striking pneumatics -- instruments made
during WW1.  The materials were in bad condition, but this after
at least 60 years after production.

I have seen one Hupfeld Phonola with skin-covered pedal bellows,
in perfect condition, 70 years later after it's production!

Leather and skin can last for a long time depending on the treatment.
I have seen organ skin from the beginning of the 19th century in
very good condition -- it could still be used.

Best regards from Paris,
Douglas Heffer