It would depend on the gas….. if using a corrosive like HCL gas to smoke someone out like 6 mins and cartridge failure is possible, if burner cs one use or about 4 hours dependent on the volume put through filter you can begin to taste it. other more exotics… without skin coverage you are as good as dead. VX phosegene mustard or sarin all go through skin and mucus membranes. they also coat surfaces after.
I have 12 filters for general purpose Hepa rated with activated charcoal backstop for .12 micron in size still wont stop smaller viruses or prions which are much much smaller ….. wonder how a virus gets stopped by latex? which is 5-10 micron in porous it doesn’t using a condom with viruses is well nearly useless since HIV is .1 micron in size.