A buried cat buried directly into the ground should decompose within a year or two.
After 2 to 3 years the cats body should be decomposed mostly to the bones and no fur or skin should be left and all that should be left of the dead cat after 5 years should be the skeleton.
If you bury the cat in a plastic container or other coffin the cats body may not decompose completely for 10 years or even 18 years.
It's best to just bury the cats body in the ground directly without the use of a coffin so that the cats body can decompose more naturally.
Bury the cat at least 3 feet down if possible to avoid other animals digging up the cats body due to the scent they can smell.