I have around a 35 gallon fresh water tank (I don't recall the exact size), and figure one week for one person with modest efforts at conservation--basically navy showers (daily) and generally not wasting water. That includes dishwashing as I don't really like eating off of paper plates.
Propane usage is quite low for me, but that's with a gasoline generator. Probably about a month of fridge, stove, and water heater usage on a tank fill (60 or 80 pounds or so nominal usable capacity...not entirely sure without looking). During cool or cold weather, with furnace usage, it would not last nearly as long. As others have said, a propane generator uses a lot of propane relatively speaking.
The level of conservation one practices varies a good bit from person to person. Still, you should have no real trouble at all going at least a week or so before needing water.