When someone dies and then the body is placed in a coffin and embalmed the body can easily last 30 to 50 years before the skin actually decays off the bones.
Without embalming of the body the body with the skin typically lasts for 5 to 10 years at most before the skin is fully gone and the bones are left.
If a body has been in a coffin for 100 years or more even after embalming the body will no longer have the skin and all you will see is the bones.
After 1,000 years or so the bones as well as the coffin may not even be there anymore.
It's hard to say though unless you dig up a coffin and body that has been in the ground 1,000 years or more.
But eventually all bodies and coffins will decay and rot away in the ground.
The body cannot be preserved in the coffin forever and that's not natural anyway.