A dust mask is a flexible paper pad held over the nose and mouth by elastic or rubber straps for personal comfort against non-toxic nuisance dusts. They are not intended to provide protection from toxic airborne hazards.
How long do dust masks last?
about 8 hours
Dust masks just don't provide a good face seal and they protect about as well as nose hair, which is to say not that well. Replace them when they look dirty or after about 8 hours of wear.