Your question was about boondocking - not dry camping in a campground with perhaps a dump station, so I'll give you an idea of how we conserved.
We carried extra silverware - go to Goodwill. Most often we used paper plates except for juicy meals or a occasional steak. We eat simply. All meats were grilled. The one vegetable pot and frypan were wiped out with a paper towel and reused. We washed dishes every 3-4 days. Dirty silverware and misc. dishes were placed in the dishpan which was stored in the bottom of the sink cabinet out of sight. When full, it was time to do dishes.
We got by using 1/2 dishpan of hot water, rinsing in the same pan sparingly. When finished the pan was dumped in the toilet. The black tank always needs water for proper dumping anyway and doesn't seem to fill as fast as a grey tank.
Hands, teethbrushing done without letting water run needlessly.
Showers - very fast wet down, turn off water, soap up, rinse. If waiting for hot water to run, catch the lukewarm and use it for heating on top of the stove for dishes or for toilet flushing.
After a while it really takes no thinking on conservation. It's automatic.
How long you can last naturally depends on your tank sizes and how many people. For the two of us we could last about 2 weeks. By that time, we were ready to move on anyway so we dumped on the way to the next stop and refilled fresh water.