How long do you have to wear a mask before bacteria and viruses grow?
Viruses that affect humans require healthy human cells to reproduce. (Well, until they damage the cell beyond being useful, or kill them.) They DO live longer on damp cloth than dry cloth, but they do not multiply there.
Bacteria, I’m not sure of — it likely varies with the bacteria. (Some may like to eat most anything organic?)
It’d likely take a few hours for bacteria to multiply in a big way in a damp cloth mask, but, if it’s damp, swap it out for a dry mask after a half hour.
If you start with a clean or disinfected N95 mask it should be fine for several hours, unless you have someone coughing big blobs of mucous all over it, or are in a very high viral load environment such as treating COVID-19 patients in a room lacking negative pressure. Dust can also clog a N95 mask, so I use a cloth mask as a prefilter over the N95. (I discovered this doing some remodeling some years back.) If the wearer has reason to think they might be infectious, an inner pre-filter seems to make sense. I’ve done that for my Mom due to me having a long running (2 months) chest cold. She’s 89+, so I am trying to avoid exposing her to even something that seems mild / non-dangerous to me.
N95 masks can be sterilized with heat: I’m now going with 150 deg. F for at least an hour. Possibly that slightly degrades the mask each time, but I have yet to see a study specifically at that temperature / time. Cloth — a laundry cycle with even a medium heat drying cycle that thoroughly dries the cloth should be sufficient. Viruses can’t take drought, so, dry them out thoroughly.
Disclaimer: I am not an “expert”, just reasonably well informed by now, I think! YMMV.