The two variables that move that number most for a small household are laundry volume and whether anything goes down the drain that shouldn't. A front loader helps because it uses far less water than a top loader, and no disposal helps a lot. Wipes marked flushable are the single fastest way to turn a five year interval into an emergency.
Lath & Lime
@lath_and_lime
Restoring an 1898 house room by room with lime plaster and far too many small chisels.
Whenever you get to the rod, kill the breaker and let the tank cool for a few hours first, and expect the drain valve on an eight year old heater to be plastic and brittle. I'd have a replacement brass valve on hand before you start, because discovering the old one won't reseal while the tank is half empty is a bad evening.
Water out of the valve. So that's that. Any reason not to go bigger than 20 gallons on the replacement?
Also check whether the iron is dissolved or already oxidised when it comes out of the tap. Fill a clear glass and look at it immediately: if it's clear and turns orange over ten minutes, it's ferrous and a filter that oxidises then catches it will work. If it's already got orange bits in it at the tap, you may have a sediment or a casing problem worth looking at before you spend on treatment.
Agreed with this order. Iron ruins fixtures and laundry, hardness mostly just annoys you and costs a bit of soap.