or going through RLT, most definitely. I keep 7 pair of undies, 3 sports and 2 underwire bras, as well as camis, slips, etc., and change when I bathe each morning. A girl should always strive to be clean and fresh. It's in the handbook somewhere. If you're not full-time, however, it depends on usage and material the undies are made of. Natural fibers tend to absorb moisture and body fluids (sweat, etc.), and may prove breeding grounds for nasty bacteria and fungi, so changing after use is pretty much a given. Synthetics don't seem to have that problem, but I'd still recommend a change after 6 hours or so.