Let's assume it's not just 0.1% but 0.001%. That still leaves us a comfortable runway of 19 million years at our current levels.
But also let's hope it's not 0.001% because I'm pretty sure the core will cool more than that on its own over a shorter timespan than 19 million years.
Wikipedia suggests cooling of 100 degrees C every billion years.
19 million years is 1.9% of the billion year rate. So it suggests the core should lose 1.9C in 19 million years. That's about 0.035% of the total temp, using Wikipedia's 5700C as the temp estimate.
That's a great initial guess, given how much uncertainty is (i've recently heard core temp could be 4x the wiki number, and some people estimate the earth's core only has lost a few degrees since it formed, rather than nearly 500 degrees). Neat stuff, thanks for providing the opportunity to look in to this.
I'm honestly not sure. I don't know how much energy is being created inside the Earth and how much is just residual. It's my understanding that the Earth is slowly cooling, though, so radioactivity isn't replenishing heat as rapidly as we're losing it.
But also let's hope it's not 0.001% because I'm pretty sure the core will cool more than that on its own over a shorter timespan than 19 million years.