How long will a paper face dust mask last before I should change it?

Rule of thumb is that you should change a paper face-mask at least after one working day (8 hour shift) when working in dusty conditions. For home users this may be a different situation, but once you can see any discoloration of the mask, discard and change and if in doubt discard and change as they are of such low cost. In all cases refer to the supplier of the masks for exact details.