Woman here. Long hair too, but jury is out on the she-devil part (husband can probably tell you). I like to wash my hair at least every 2 days, even when boondocking, especially since we like to do RZR rides while boondocking and things get pretty dirty. I am very careful with water usage elsewhere so I can do this... ie use disposable cookware, drinkware and dishes so I only have to do dishes every couple days. Brush teeth outside using bottled water. Could even do dishes with bottled water using a tub outside if needed to conserve both water and gray tank capacity.
I also have it down to a science... wet hair just enough to be able to suds the shampoo (could do this part with a bottle of water outside prior to undressing and getting in the shower). Turn off water flow. Get the suds going, wash the rest of me while I'm at it. Then rinse all at once. I've never measured how much water I use, but I'm guessing I don't use more than a gallon or two.
Edit to Add: I also use cheap Suave shampoo vs the more expensive stuff I use at home. It suds less.