My hair is quite long and really thick, which does become challenging when camping. At home, I wash my hair at least every second day.
When camping we are usually near the coast, so if it is warm enough, I will swim and wash my hair in sea water using Peter G's Night & Day Shampoo (it lathers really well in salt water but smells a little chemically). Then I generously apply a cheap supermarket conditioner and give my hair a good coat with this as the N&D shampoo can be a bit drying + salt water. We also use the Night & Day Liquid Soap to wash in seawater. Then just a really fast freshwater rinse from limited water supplies we have on board.
In winter when the ocean is too cold to swim, I TRY to go 3-4 days between washing my hair, but at least I wash my face and fringe (with shampoo) daily so it feels clean and fresh. On the second and third day, I usually just tie my hair up in a pony-tail or bun and live with it as is. After all, it's camping and a cap/hat can hide it. The dry shampoo doesnt work for my hair as it is too thick and I find it just makes my hair feel caked and horrible.
That routine seems to work for me, and husband has less hair every year ... so needs less water for washing his hair and that means more water for me
If we have plenty of water or know we dont need to ration supplies then I stick with washing my hair every second day. Hair dryer stays home, as in summer I am always wearing a hat and in winter a beanie.
I have always said that eventually when we take off for our indefinite lap of honour around Oz, I will cut all my hair off so that it is easier to manage and less water needed.