If you sweat and you don't use soap and you don't want to stink you need to wash every 4 hours or so. If you don't sweat you need to wash every 8 hours or so. Soap adds about 50% to those numbers.

If you're not dancing closely with people maybe you can extend that time a little bit, but those are the numbers we settled on as a dance community who regularly had dances longer than 4 hours...