Comment by mindslight

I've used the same 3M 6011 (organic vapor) filter for years, for tasks like cleaning with ammonia where I'm not really worried about hazardous exposure just nuisance [0]. And it's lasted just fine (extremely off-label use, of course). So with something like tear gas where you're not worried about actual toxicity per se, I'd think that subjective experience would be fine for knowing when it needs to be changed. If you want to be paranoid, it seems like you could also carry a backup fresh set still in the sealed foil.

With the prices for respirators going up (/me gestures at federal "government" attacking our own cities), if I wanted to save money I'd get a knock-off full face respirator from Amazon (or Ali if you're comfortable transiting the import blockade with this specific item), and then genuine cartridges (NOT from Amazon obviously, lol).

[0] for most things I do, I use the 2097 filters which are primarily particulate with just a small amount of activated charcoal for "nuisance vapors". This is all generally with half-face 7500 bodies.