The original cast iron distributors haven't been available as spares for a long time. This makes all of the ones in service very old.
Among of the deficiencies the iron units corroded the plates between upper and lower chambers. The part is really a very durable component that loves constant use. I would say that sitting would be the greatest source of damage. In continuous use it can last forever.
I don't think they ever wear out. They corrode or get restricted with trash.
Bosch Master, ASE Master, L1
33 years MB technician