Disposable dust masks are designed for single use only, after each wearing they should be discarded. A single-use should not exceed eight hours.
However, the mask may not last this long in heavy concentrations of hazard, when the mask becomes clogged the hazard will start to be drawn in around the edge of the mask, the easiest way in, so it is very important that they are not used for too long.
Used masks should be disposed of safely as they will have the properties of the hazard that have been used to protect against.