I speak from observation only. I have never read of a particular time. I have seen the results of filters that sat but have no feel for the time frame. I have seen filters with the paper crumbling, I have seen filters with the pleets stuck together, I have seen paper glazed from either hydrocarbon vapors or treating chemicals till they were impervious.
I would not like to use filters I knew to be over 2 years old, but that would be my factor of safety. I have no idea whether they don't spend that long in normal distribution, but I doubt it.
Bosch Master, ASE Master, L1
33 years MB technician