The time it takes for a body to fully decompose depends on if the body is exposed to the air, in a coffin, has been embalmed etc.
For a body that is not embalmed and exposed to the air the body can fully decompose in a few weeks to a month or two months depending on the conditions.
In a coffin a non embalmed body can take a year or 2 years or so before the body fully decomposes.
For an embalmed body in the coffin it can take 10 years or even 20 years or more for the body to fully decompose.
By 50 years in, your tissues will have liquefied and disappeared, leaving behind mummified skin and tendons.
Eventually these too will disintegrate, and after 80 years in that coffin, your bones will crack as the soft collagen inside them deteriorates, leaving nothing but the brittle mineral frame behind.
In a temperate climate, it usually requires three weeks to several years for a body to completely decompose into a skeleton, depending on factors such as temperature, humidity, presence of insects, and submergence in a substrate such as water.