How long does it take to turn plants/animals into fossil fuels?
As little as 6 months.
Don't be confused by “fossil” fuel labels. The fossil fuels of coal and oil took many years to form through a combination of chemistry, time and pressure. But this question is confusing the source (previously living organic material) with a time label (fossil). Modern methane gas production from landfills can start up in a few months after sealing a landfill, and continues for decades, depending on the size of the landfill.