There is no magic number.
When the logic board in most thermostats determine the heat pump can't keep up (system runs too long) or a big offset needs satisfied (moving the thermostat from say 60° to 80°) then it will typically switch to furnace.
Also, outside humidity has a lot to do with how much heat can pe pulled from a certain ambient temperature. The conditions of getting heat from 40° isn't always the same.
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