The rate of decomposition is a function of temperature and moisture; the warmer it is the faster the biological factors (insect and bacterial) work. A moist environment--up to a point-- facilitates these activities (submerged in water precludes most insect activity, as well as usually reducung the temperature). Thus the climate, season of the year, and physical exposure to the elements all have a big effect on how fast a carcass will decompose. If it's hot and humid, decomposition can start to be evident in a week; cold and dry, it may take months.
