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.