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.
You might try to switch your respirator. The Aura series (3M 9210+ being the best Aura, IME) and VFlex (3M 9105) seem to be very comfortable and easy to breathe for me & others who I've shared them with. In general, a larger surface area makes it easier to breathe.
Another option is to look for a respirator with an exhalation valve. You give up a level of source control but any exhortations to "protect others!" is pretty silly when they come from people who aren't wearing anything to protect themselves. The Aura has a vented model (9211+) which I've not tried but have heard excellent things about.
You could also consider a half-face elastomeric respirator. You wouldn't expect it, but I've found the ones made with a quality silicone material (3m 75xx / Honeywell North 7700) to be far more comfortable than disposable N95s. They have far more breathable area, which is a major plus. When adjusted properly, they also do not "dig into your face" the same way disposable respirators do - you can keep the straps surprisingly loose & still maintain a seal. And they have a much lower TCO because the respirator media lasts for a long time. Muffled speech is the biggest problem, but speech diaphragms go a ways towards fixing that.
In general though, as you move up the ladder of protection you end up using tools that are both more effective & more comfortable. Pre-pandemic I had a crawlspace under my house spray foam insulated. A team of three guys crawled around under my house in mid-August while wearing full face respirators. They told me they were actually more comfortable than the old half-mask / goggles combination they used to use.
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.
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.