Expiration Date

The Question is: “How often does one change his or her underwear??????........now don’t get grossed out…this is purely hypothetical…while we are on the topic of “travel”…. one may not have access to laundary facilities or may have forgotten some things…..now, is it so wrong to wear your underwear more than once?  Or does it depend on the level of stainage?....just a hypothetical.