LOL ... I guess you wouldn't last too long out here then ... It is nothing to go two weeks between washes and if you got so much spare water to wash your knicks you are not on your bike. IIRC 11 days, two pairs of knicks no washing is my "record" and heck I am still alive

. Fourteens + days coming up including carrying water for five of those days (read at least 30 litres). None of that is going towards washing clothes trust me.
If by chance I hit a water hole I will rinse clothes but otherwise nothing happens until I hit a town with a washing machine. On the hand I try to where possible have a bush shower if there is a waterhole or other substantive water supply such as rain tank at a campsite. In those situations I use 200 to 300 ml of water warmed up to wipe myself down. That is a luxury on some days.
Regards
Andrew