N95 masks still work (substantially reduce spread when worn by index patient and/or their contacts) when not fitted carefully. They then don’t live up to their rated specification, but are still a whole lot better than nothing or a lower-filtration mask.

Not sure how to parse the question; answering both ways.

How long can a person wear one? The few times I work in office I wear one all day, and don’t find it difficult.

How long can a single N95 be reused? I’d read research indicating the real limiting factor seems to be the quality of the seal, and the straps tend to fail before the mask loses efficacy. Basically, if it seals without air escape, you’re good.

they were designed to be disposable, but when they were in short supply at the start of the pandemic, medical professionals were wearing them up to a month. There was a study that you could treat them with, I think, UV light, oh no wait, it was put them in the oven at a certain temperature, to extend their viability up to a certain point, but I think practically, most people switch off every week or so unless in extremely high risk situations.