The length of time that it takes for a dogs body to decompose is from a couple of months to a year.
Once a dog dies they will begin decomposing on the first day but it can take a month or two for small dogs to decompose fully and for larger dogs it can take as long as a year to fully decompose when buried.
When above ground the dogs body can decompose fully within a few days to a week for smaller dogs and larger dogs.
Dogs often decompose slower when buried underground due to the lack of oxygen and when above ground the oxygen can accelerate the decomposing of the dogs body.
The presence of oxygen makes a dog's body decompose faster as decomposition is a natural process that involves the breakdown of organs and tissues by microorganisms and oxygen is essential for these microorganisms to thrive and perform their work.
Bacteria and other microorganisms that decompose the dogs body require oxygen to survive and carry out their decomposition activities.
The more oxygen that is available to the dogs dead body the more rapid the decomposition will take place.
When a dogs body is buried underground they don't have a lot of oxygen so the body decomposes slower.
But in most cases a dogs body will decompose within a few weeks or less when above ground and within a month or two for small dogs underground or up to a year for larger dogs buried underground.