Millions of years. Much of the coal we use is made of plants that died 100 to 400 million years ago.
Coal usually takes millions of years to form. The highest quality of coal is from the carboniferous period, which was about 360-290 million years ago. As the number of years that it has been in the ground decreases, usually the quality of the coal decreases, however in extreme conditions it can be brought into maturity much faster. There is usable coal from about 1 million years ago, but that is about the earliest that coal can be mined and used.
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