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...

Core of earth: 6,000 ºC

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://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

We could just balance the system:

core of Earth too hot -> use geothermal

core of Earth too cold -> use solar power

We basically can not accelerate the process. Almost all the resistance to heat movement is on layers we can not reach at all.