How often does the mask change?

Due to different types of masks, different groups of users, and different environments, it is not possible to uniformly specify how long masks are used. Under normal circumstances, the disposable medical surgical mask is recommended to be replaced at least 4 hours; if the mask is wet or dirty with secretions, or is damaged, or feels increased respiratory resistance, it should be replaced immediately.

Medical Standard Respirator Disposable N95 Dust Mask (14)