1) If you're just cooking, the propane will last a very long time. Months? If running the furnace, it might last a week or two depending on how cold it is, and how big your propane tanks are.
2) If you have full hook-ups, you can shower and use the toilet as long as you want. You can leave the grey water (sink/shower) valve open, but leave the black (toilet) valve closed until at least 2/3 full. If boondocking (no hookups) the gray tank will fill up fast if you take normal leisurely showers.
3) Google