How do you decide when to change N95/FFP-2 masks?

General Discussion:

Broadly speaking, there are five issues to worry about when reusing masks:

1. Virus particles contaminate the mask's surface, and may spread to you while handling the mask.
   • Mask filter surfaces (where the air flows through) may have spent hours collecting COVID particles from the air, since you've been continuously pulling contaminated air through the mask. The filter surfaces may have hundreds or thousands of times the contamination seen in solid surfaces.
   • Reaerosolization of filtered particles (where particles trapped by the mask re-enter the air) is possible, but likely releases negligible amounts of virus (source, source) compared to the ~5% an N95 mask fails to stop.
   • There are a number of approaches for decontaminating masks. For coronavirus specifically, the simplest approach is to just let the masks sit. The time necessary to inactivate COVID virons depends on the temperature and humidity. Options include: (source)
     • 4 days at 21-23 °C, 40% humidity (However, this source indicates virons may be present after 6 days)
     • 1 hour at 70 °C, any% humidity (using, e.g., an oven)
     • Boiling water for 5 min (may lose ~8% filtration efficacy, but also cleans mask of dirt)
   • UV-C radiation can also decontaminate masks. However, this process is potentially unreliable because the UV intensity needed varies with mask material, masks with unusual geometry may shadow portions of the mask from treatment, and dirt or other soilage may block radiation (source). Make sure to use >= 1 J/cm^2 for >=1 minute (source). Don't use > 10 J/cm^2 to avoid damaging mask structure.
   • Chemical agents such as ethanol and bleach may reduce mask filtration (source).
2. Loss of mask structure prevents a good fit to your face.
   • Generally, it's hard to properly fit an N95. Among 74 anesthesiologists, 63% of women and 29% of men failed fit testing, even with a fresh respirator (source). Overall failure rates were 43% after 4 days, 50% after 10 days and 55% after 15 days. Additionally, people were very bad at estimating the quality of their fits, with 73% of those who failed the test thinking they had a good fit.
3. Loss of electrostatic charge worsens filtration efficacy.
   • N95 masks don't lose much efficacy if they're just stored, even for years at a time (source). However, they do eventually lose efficacy if they're actually used. With 8 hours per day of use, N95 masks retain ~95% efficacy after 3 days, ~92% efficacy after 5 days, and drop to ~80% efficacy after 14 days (source). Note: this refers to just the material's filtration efficacy, and does not take into account any further reduction due to worsened fit quality.
   • Mask electrostatic charge degrades more quickly in humid environments (source). Thus, a mask with an exhalation valve will likely last longer. An N95 respirator with exhalation valve is likely as effective at source control (preventing spread from you to others) as a cloth or surgical mask (source), but many establishments (such as airlines) do not allow masks with exhalation valves.
   • If you want to get fancy, this paper describes a procedure for recharging a mask's electrostatic potential. However, that won't help with the loss of structure issue.
4. Accumulation of filtered particulate makes the mask harder to breathe through and makes inhaled air more likely to pass around the mask rather than through it.
   • I don't think this is usually an issue because loss of structure/efficacy will force you to change masks more quickly than the masks get clogged. However, if you're in a dusty/smokey location, it could be a problem. I suggest changing out a mask as soon as you notice it getting more difficult to breathe. You can also wear a surgical mask over the N95 to protect it from larger contaminants.
5. Accumulation of sweat/dirt/etc makes the mask disgusting to wear.
   • I suppose this is up to personal preference.

Final Recommendation:

I'd suggest replacing an N95 mask at least once every 5 days, and preferably once every 3 days. I'd suggest 1 hour at 70 °C for decontamination. Additionally:

• I'd recommend using a mask with an exhalation valve, if you can.
• I'd recommend storing masks in a low-humidity environment while not using them.
• It's best to handle potentially contaminated masks by the straps, especially during removal. If you absolutely must touch the mask itself, avoid touching filter surfaces or interior, and instead touch the edge of the mask somewhere that's away from your mouth and eyes.
• Virus can still transfer to your hands, even with proper handling (source). You should wash your hands after handling a potentially contaminated mask.
• You should never stack potentially contaminated masks. I.e., don't allow the filter surface of one mask to be in contact with the interior of another.
• Wearing a cloth/surgical mask over the N95 will help protect it from splashes and large contaminants. However, this may accelerate the loss of electrostatic charge by increasing the humidity within the mask. Do this if you think your N95 may get spoiled otherwise.
• If decontamination is a chore, one option would be to use a rotating set of masks, wearing one a day sequentially until you're worn them all once, then decontaminate the entire set using an oven.

Additionally, you may want to consider alternatives to N95s. Half-face elastomeric respirators are designed to be reusable, are far more protective than even a properly fitted N95, are much easier to fit properly, and I personally found them much more comfortable than expected. Additionally, they only require replacement filters when breathing becomes difficult, so they cost less in the long run.

Finally, at the highest tier of protection, you can buy powered air purifying respirators for $300 or make your own for $15-30. I don't have any experience with either option, so I can't comment much.