If you bury the dog directly into the ground without a coffin the dogs body should begin decomposing within 6 months of burial and then after 1 to 5 years the dogs body may be completely decomposed to the bones.
However if you bury the dogs body in a coffin it can take around 10 to 20 years for the dogs body to completely decompose.
I've always just buried any dogs I've had in the ground without a coffin so that there body can decompose faster which is natural.
Bodies are not meant to stay around for long after death and the sooner the body decomposes the better.
Some places may require a coffin for dog burial while other places you can just bury the dog directly into the ground.