Hygiene:

If disposable: I generally rotate a disposable after each long use, let it air out for a couple days and use a different one. Honestly, the headstraps tend to break just from repeated donning and doffing and that prevents overuse. But if the straps don't break, eventually a disposable respirator may get beat up or kind of visibly gross and then I toss it and use a new one.

If elastomeric: Disassemble and wipe down with a bit of water every few uses or after a long use. It depends. Wash it occasionally. The biggest problem I ran into with my elastomeric was that it traps a lot of water and I forgot to disassemble and dry it after use, let it sit for a few days, and came back to find it was really musty. Disassembled it and washed it but the mildew was growing in the filters so I had to just toss the filters and get a new one.

Other than that there isn't really a concern. How quickly a respirator will get gross will vary based on your activities while wearing it and how long you wear it. If you're working a physical job in a respirator all day, you may want a fresh one every day just because you don't want to be pushing old sweat and body grime onto your face at the seals.

Environmental: I mean, we are just individuals trying not to get fucked up by the plague. Our impact is minimal compared to the environmental devastation that Capitalism and its related imperialist wars drive. I don't worry about it, except for one thing: When I'm done with a mask I always rip the straps on one end so that if some animal encounters it in a landfill or the wind picks it up, they don't get tangled in the straps. A thing I read about back in 2020.