Are you talking about a tank inside a house or an outdoor pond?
Inside a well-insulated house, I reckon you'd almost get away without a heater year round. I'm in Canberra, so considerably colder, I turn the heaters off November-ish and leave them off until late March/April depending on the weather. Yesterday my two tanks were both sitting at 22 degrees.
A few factors to consider would be, size of the tank (large tank = larger thermal mass = less fluctuation), thickness of glass, and house's insulation.