It's worth keeping in mind that when coal reserves were first being estimated in the US, ~1865 - 1875, the total reserves were considered to be over 1 million years' supply at then-current rates of usage.

Oddly enough, constantly growing usage can cut into that time-to-depletion estimate fairly quickly. Present estimates are that US coal reserves are ~200 - 1,000 years, depending on the recoverable yield.