For just one person, maybe 7-8 gallons for drinking. For bathing, if it's just you, 2 showers should be enough. One at the end of week 1 and the second before you head back to town. Use baby wipes on other days to stay "fresh". I use about 2 gallons for a shower, I wet up, turn off water and soap up, then rinse. Many people use less water than that for showers. Depending on cooking, I cook mostly with rice(instead of noodles) so that can be calculated. As to washing dishes, I read a great tip for using no soap but use spray bottles to clean dish, then a vinegar/water mix spray for sterilization. I think 15 gallons would last me 2 weeks easily enough and could prob do it with 10. 15 gallons is about 125 pounds.
Maybe buy the drinking water(here it's a buck or less for a gallon), and then a seven gallon jug from wally world for the other water needs.
After you get back, I would like to hear what your experience was with the water. So far, 3 days is the longest I have been without a tap nearby. I am planning on 2 week excursions myself when I finally get out west.