About as long as it would take to cool the surface of the sun. https://www.forbes.com/sites/alexknapp/2013/04/28/the-center-of-the-earth-may-be-hotter-than-the

About as long as it would take to cool the surface of the sun. https://www.forbes.com/sites/alexknapp/2013/04/28/the-center-of-the-earth-may-be-hotter-than-the-suns-surface/?sh=29ad0ff628e9 https://www.forbes.com/sites/alexknapp/2013/04/28/the-center... Core of earth: 6,000 ºC https://www.space.com/why-sun-atmosphere-hotter-than-surface https://www.space.com/why-sun-atmosphere-hotter-than-surface Core of sun: 27 million ºC Surface of sun: 6,000 ºC "The leading idea among experts is the sun's magnetic field is actually bringing energy from inside the sun up through its surface and into its atmosphere." The "napkin math" will involve gathering some data about RF power harvesting, and some assumptions (e.g. cosmic rays) https://news.mit.edu/2020/energy-harvesting-wi-fi-power-0327 https://news.mit.edu/2020/energy-harvesting-wi-fi-power-0327 https://en.wikipedia.org/wiki/First_law_of_thermodynamics https://en.wikipedia.org/wiki/First_law_of_thermodynamics Do you believe in the 1st law of thermodynamics, that energy is neither created nor destroyed? https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe We could just balance the system: core of Earth too hot -> use geothermal core of Earth too cold -> use solar power