In my case it is based on TDS, hardness, foam, waterline, filters -how often they need to be cleaned and how dirty they are when they ARE cleaned - and the amount the tub gets used.
Add that all up, divide by two, carry the one, and I have recommended three or four months for everybody.
Since we don't have freezing conditions or snow around here, I tell people the more you change your water, the longer you'll live, the better you'll look, and the more money you'll make. Seems to convince 'em.