I used to mow 1/2 acre of Bermuda grass at least once a week from May - October on a riding mower. Would take 2 1/2 hours. I used the n95's you get at Home Depot and would reuse them for many weeks and probably could have continued to reuse them until I couldn't breath through them or the straps broke. They're for stopping particulates so as long as they're not clogged they still work. If the particulates are infectious virus particles then reuse becomes more problematic but they still filter particles.

Self reply to all others: try doing something that actually tests the limits a bit for health reasons. you can tell in a welding/woodshop when N95s and P100s go bad, they get physically harder to breath through after about 6-8 hours. If they don't they're not sealed well enough. they do keep working longer (probably), but they get quite a bit worse.

The filtration doesn't run out when working with particulates, it actually gets better. The reason you have to replace them is they get too good at filtering and it restricts natural breath (or they get damaged).

In this situation, you assume the mask catches particles containing coronavirus. The mask becomes a biohazard pretty quickly. They could probably be safely disinfected and reused.

Wore mine for 2 weeks during the fires when they were in short supply. It was definitely worse for wear at the end. And mine didn't have a valve on it. If it did, it might have lasted longer.

As far as I’ve heard, this is what the label recommends; however, in practice, they can be used for much longer.

(I cannot remember my sources, sorry.)