My experience is obviously with the m40 promask coupled with NBC company defense training followed by MSA SCBA use/training in the civilian sector as a firefighter.
One day's growth was the rule for firefighting... any more than that and you're at best support, at worst you get left at the house. We did an in-house study with 5 people of various beard quality capability and had them re-qual with their originally, shaven face, sealing SCBA mask.. and after 2 days 4 of the 5 couldn't maintain a proper seal... this was using the same sort of machine testing based (move your head around talk breathe do flutter kicks jumping jacks now talk dirty to me baby ok that's too dirty pushups go) that I used for testing or was tested in the army. The banana oil/smelling salts test is inadequate.
Obviously clean shaven has a huge bit to deal with just "professional appearance" as well as a visible personal hygiene indicator for those who may have slovenly troops. The reason we did the time lapse seal testing is simply because volunteer firefighting is like a permanent QRF... middle of the night, wake up, save the day... being able to have shaved in the last 48 hours actually would make response time faster on average by 30 seconds... I still was running my electric razor scraping my face on the way to the station, it took me 3-8 minutes depending on traffic and since the road was straight I could get away with it. Plus, a structural fire is just as hazardous as a CBRN environment, it just kills you in a more painful and drawn out manner if you didn't pay the fuck attention, compared to say VX.
