As already very much mentioned, it depends on your usage. But for reference, we do a boondocking trip in hot weather every year for 5 days. We never come close to running out of water or filling up the gray, but we have a 112 gallon tank. I'd say we might use up 1/3 of the water if we arrive with a full tank. Gray tank is 44 and we maybe use up 3/4 of that. I navy shower every day, including hair washing, husband will every couple days. We use paper plates/plastic utensils, the only thing I wash/rinse out most of the time is the coffee cups and percolator. Our black tank is 44 and it gets on the full side. But I will add that my husband has Crohn's disease, and well, needs a lot of water to flush sometimes, and we both drink a lot (both alcoholic and non-alcoholic beverages) on this trip, so the toilet gets a lot of use.
We also run the generator (built in Onan 5500) for 10 hours a day to power 2 AC units, but it's hard to gauge gasoline usage because we just top it off whenever us or someone in our group runs to town and fills up some gas tanks. We don't keep track because we all take turns.
Personally, the only thing I really worry about running up to the limit on is the black tank, because that is hard to dispose of and you can't exactly stop using the toilet if it gets full. But, we have once made it to 7 days boondocking on Forest Service land without dumping... husband used a bush for #1 during the day on that trip though :)