The idea is to sterilize after every outing.
I think people are confusing these with the masks worn by medical staff. The people in hospitals [etc] are wearing masks (and other PPE) to give some degree of protection from the infected moisture exhaled by infected persons in the room. Their PPE has to be changed every couple of hours and sterilised before re-use to remove the infected material which has landed on it and soaked in.
The public are wearing masks to provide a degree of protection to *everyone else* by trapping the moisture they exhale - in case that person is infected. As long as they are not walking around in high-risk areas (eg hospitals) the wearer is already infected with anything the mask has trapped. As such the only reason for washing it is to reduce the risk of transferring any trapped liquid by handling it. As such there is really no point in trying to "sterilise" (or even clean) masks more than about once every 2-3 days.
AS
