When to replace respirator filters?
You are correct regarding the dust filters... in fact the &quot;caking&quot; particles can sometimes improve filtration... so the cartridges only need to be replaced when damaged, soiled or when they unacceptably obstruct breathing.
As for chemical filters... this depends a great deal on the chemicals being dealt with, as each will have different properties.
Each cartridge should have a specified service life or change schedule... nonetheless, for vapors and certain organic compounds 3M™ recommends that they be used for only a single shift exposure

Can chemical cartridges be used for more than one shift?
Organic vapors are removed by the process of adsorption.
Weak physical forces hold the organic vapor on the
activated carbon. Since these forces are weak, the process
can be reversed and the organic vapor can be desorbed.
Desorption during storage or nonuse periods can result in
the migration of the chemical through the cartridge.
Migration is mainly a concern only for organic vapor
cartridges. Organic vapors adsorbed on an organic vapor
cartridge can migrate through the carbon bed without
airflow. Desorption of very volatile contaminants can
occur after a short period (hours) without use (e.g.,
overnight). Partial use of the chemical cartridge and
subsequent reuse could potentially expose the user to the
contaminant. This is most significant for the most volatile
and poorly retained organic vapors (e.g., boiling point &lt; 65°C). For organic vapors with a boiling point less than
65°C, it is recommended that the organic vapor cartridge
never be used longer than one shift even if the estimated
service life is greater than 8 hours and the cartridge is
used for only a short time during the shift.