Peat, brown coal, soft coal, and anthracite are fairly clearly formed from vegetative layers. Some of that is very ancient and it is constantly being formed even now. The surface of the earth is constantly being buried by dust, including meteoric dust, maybe an inch a million years. Whatever vegetable matter is present on the surface will eventually be buried and will eventually go through the coal formation process. However, we can dig it up and burn it far faster than it forms naturally. 500-1000 years is about all we can expect from this energy source.