We know that if the convection in the outer core stops, the Earth's magnetic field stops, and removing enough heat from the core will stop the convection.
We know how weather works quite well, but knowing if it will rain in a week is an entirely different beast.
My memory is that the calculation found that if humanity switched to geothermal for all its energy needs, then in only about 1000 years, the core cools enough for the magnetic field to stop, but I am not sure.
(We should definitely deploy geothermal in the Yellowstone caldera though long enough to cool it down enough so that it will not erupt again.)
Humanity could max out geothermal for a million years and never make a dent.