If you go strictly by the label, if worn properly and cared for, n/kn95 masks are usable for up to 40 hours (non-consecutively). There are many factors that can shorten their lifetime, like the mask becoming wet, damaged, and or soiled. The most obvious sign it’s time to replace your mask (other than 40 hours of use) is if it no longer seals properly. Performing a seal check is crucial. Here is an example for 3M Aura N95 masks.

There are purported ways of extending the lifetime of masks, though I don’t post any of those because they can cause a lot of disagreement. But I will confidently recommend not putting your mask in a microwave or oven to sterilize it.

Elastomeric respirators with replaceable filters are one option - I got the 3M 7502 and it is the most breathable respirator I’ve had the pleasure of wearing, more so than the disposable Auras we had been using til I got it. All said it was about 30 bucks for the face part and 25 for 3 pairs of filters that snap on to it which are labeled for 40 hours of continuous use. Our local respirator expert says since those are specifically rated for things like sawdust, ash etc. trying to get through, if you’re only using it against viral particles and residual like smog or whatever then you can stretch that out for much, much longer- she estimated about 3 months per pair before needing to replace them where I live on the Pacific Northwest US, and I would trust this woman with my life when it comes to this stuff.

In general I can’t recommend elastomeric respirators enough. They’re much less wasteful and way better protection overall. The one I have that I already mentioned makes me look like she-bane but is a p100, a rung above n95 protection which is already stellar. The filters I use are hot pink, purely coincidentally, which is my favorite color lol. The Flo mask is a popular lower profile option (but it is more expensive) if your work would get pissy about the more intense PPE. My partner does this since she works retail and her bosses suck, but I can’t work and no one will hire me so I’m more than ok turning a few heads on my grocery trips and scaring the shit out of a few racist meemaws.