How long you can wear an N95 mask depends on what you’re doing, and how long you wear the mask per day.
Ideally disposable N95 masks should be replaced every day. This is the standard policy for businesses for which N95 masks are required. (I speak from personal experience, having worked at industrial plants across Canada.)
For personal use, it’s subjective.
Case in point: when I lived in China, I wore N95 masks nearly every day I left my apartment. I didn’t have many of them on hand, and getting authentic 3M N95 masks was challenging, so I wore each N95 mask for about a month. Mind you, I didn’t do any exercising in the masks other than walking around town. Had I been running, I would have been breathing a lot heavier, requiring the masks to be replaced much more often.
The N95 masks have an elastic that wraps around the head. Once they get stretched out too often, or too many times, the elastic becomes too loose, making it difficult to maintain a tight seal over the mouth and nose.
It is probably a good idea to cycle through multiple masks during the course of a week to maximize the life of their elastics. What I mean by that is that you should have one mask reserved for wearing on Mondays, one mask reserved for wearing on Tuesdays, etc.
By waiting several days until you wear the same mask again, you slow down the rate at which the elastic gets stretched out and loses its effectiveness. That is because the time when the mask sits idle in your home is the time when the elastic fibers can naturally relapse to the tension they’re intended to be at.
If an elastic is regularly in a state of being stretched and doesn’t have enough ‘down time’ to return to its natural state, the elastic fibers adjust. This results in a mask whose elastic loses more and more of its elasticity.
The simple but effective process of safeguarding against PM2.5 masks having their elastics ‘stretched out’ prematurely is something I picked up once I became serious about swimming and exercising, ditching loose attire intended for amateurs for tights intended for serious athletes. People who wear stretchy articles such as swimming briefs, leggings, bras, form-fitting underwear, etc. would benefit from cycling through them instead of wearing the same article on two or more consecutive days.