For those I usually just replace them when they look dirty. If you feel restriction of the air flow, replace them sooner. Those particular filters are cheap enough that it's doesn't hurt to replace them more often than you think you should. If you ever use the carbon filters, those remove the odors from organic vapors, so replace those when you can smell the vapor. Also, store the carbon filters in an air tight container of some kind, they absorb things even when sitting around. Give that mask a thorough cleaning with rubbing alcohol and check for tears in the rim.
If it was used for lead removal, I'd recommend pitching the entire unit. Respirators are relatively cheap. Lead is bad news and there is a reason for the new EPA Lead-Based Paint rules. I've dealt with LBP Removal teams who had employees with elevated blood lead levels who also took lead home to their little (<4) children. Not a pretty thing.
If you want to keep it, remove the filters and scrub everything with TSP (Trisodium Phosphate) which you can get from Lowes/HD. It will chemically bond with the lead to remove it properly and actually "clean" the mask.
When it comes to changing out filters with wood dust, just go by restriction of breathing or every "x" number of months based on usage.
As for the Resp-O-Rator, I tried one and it's not bad, just takes some time to get used to. My problem was breathing through my mouth like SCUBA equipment.
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