According to 3M's official website, there is no regulation on how long medical surgical masks should be worn at a time. It just says "When the mask is dirty or damaged, or breathing is difficult, leave the contaminated area and replace it with a new one."
According to the instructions of the Center for Disease Control and Prevention, practical experience shows that medical surgical masks can be used continuously or intermittently within 8 hours. Whether the surgical mask can be used for a long time depends on whether the medical surgical mask is dirty, whether it can be sealed continuously, and whether it affects smooth breathing; Reuse medical surgical masks should pay attention to cross-contamination. The maximum safe re-use times of medical surgical masks cannot be determined due to different circumstances.
The CDC also stated that it is better to use medical surgical masks for a longer period of time than to take them off and put them on again, which can reduce the possibility of contamination. Especially in an environment where there are epidemic germs in the air, if the mask is removed, it is difficult to ensure that the germs will not stick to the inside of the mask.