How are fossil fuels formed in the earth and how long does the process take?

Fossil fuels are organic materials formed in the earth as a result of slowly decomposing plants and animals. It takes millions (sometimes hundreds of millions) of years to obtain fossil fuels and this is why they are regarded as non-renewable source of fuels.
Fossil fuels comprise of crude-oil, coal and gas. In general, decomposing dead plant material form coal while crude oil and gas are formed from dead marine organisms.
Several million years ago, a large number of ancient living plants and animals died and their remains were buried in mud. As the years went by, additional layers of mud sediment were deposited until rising temperature and pressure beneath the earth compressed and converted the mud into rock while the dead plants and animals slowly decomposed into fossil fuels. Greater detail on the processes leading to the formation of each fossil fuel can be obtained in the reference link provided.  
https://www.energy.gov/fe/about-us/students-and-teachers