An additional dumb question. If all the expense was undertaken to build all the piping to functionally turn the tube into a heat source for buildings, how long

An additional dumb question. If all the expense was undertaken to build all the piping to functionally turn the tube into a heat source for buildings, how long would it take to sap the stored heat from the soil to the point where the temperature differential doesn't work anymore? At some point would they have to switch from having it be a heat source in winter to making it a heat dump in summer?