X100 masks are TESTED at 0.3 microns as that's the size particle that most easily penetrates them. Same goes for X95 and X99. The reason I use "X" is that there are N, R, and P masks, all of which are tested for their ability to filter 0.3 micron particles to a specified level. The N/R/P designation is for their resistance to oils. An X95 mask is certified to filter 95% of those particles. An X99 mask is certified to filter 99% of those particles. An X100 mask is certified to filter 99.97% of particles. Here's what is not often told to you: many X95 masks will actually filter between 99% and 99.97% of the particles they're tested against, so most could qualify as an X99 or X100 mask but because the batch tested may not entirely filter at that level, the batch can't be certified to that level, so they go to the lower standard instead. The P100 filters you have are considered "oil Proof" (therefore the P rating) and will filter particulates (0.3 microns) 99.97% all the time, thus the "100" designation.
The manufacturer will warrant that performance level for 6 months or 40 hours of use, whichever comes first. After that, you're on your own. Some filters will retain their performance well beyond that. Some won't. Once you go past 40 hours (cumulative) use, you're on your own and you could continue using the filter until breathing becomes difficult (clogged filter) or it becomes physically damaged and therefore physically is unable to filter incoming air.