Heat passes from the Earth's core through the mantle to the crust at 45 TW. That heat can be converted to usable energy (aka exergy) at 70% efficiency. So global geo-thermal energy production is limited to 32 TW. A given geo-thermal plant slowly cools the rock below it and produces less and less energy until it reaches equilibrium with the flow of heat energy into the rock it draws from.
See page 20 of https://gcep.stanford.edu/pdfs/DyUMPHW1jsSmjoZfm2XEqg/1.3-He...
And https://gcep.stanford.edu/research/exergy/resourcechart.html
Nuclear decay and fission produce 30 TW in the earth's core and 45 TW flow out to the crust, so the earth's core is gradually cooling. The core contains 1e31 J of energy and is currently cooling down at a rate of 15 TW = 1.5e13 W. A Joule is 1 Watt-second. Therefore the earth's core will take longer than 30 billion years (1e18 seconds) to cool down.
See https://en.wikipedia.org/wiki/Geothermal_energy#Renewability...