The length of time it takes for a dead body to decompose in a coffin depends on the coffins construction and whether or not the body was embalmed.
If the coffin was made of wood and the body was not embalmed then the body could decompose in only 5 to 10 years.
If the body was put in a metal casket and the body was embalmed it could take 10 years for the body to partially decompose and then another 10 years for the body to fully decompose.
Then again in some rare cases it could take as long as 30 to 40 years for a body to decompose after it's been embalmed and buried especially in a metal casket.
Bodies are buried in vaults along with the caskets and being embalmed so it can take longer than normal for a body to decompose.
There have been bodies dug up and moved to different graves 50 years later and part of the body was still intact although the body was partially decomposed and didn't look too good.