How long can medical masks be used continuously?

How long can medical masks be used continuously?
Medical protective masks are basically limited or disposable and cannot be reused after washing.
At present, the 3m1860N95 respirator is the most widely used medical respirator in the market. It is used in outbreaks such as avian influenza, Ebola, and Middle East respiratory syndrome. It can normally last for 8 hours. However, once the medical respirator is contaminated with blood or saliva spatter, it must be replaced immediately.