It really all depends on the conditions of the environment the body was entombed in. Many different factors, from the quality of embalming, to the materials the casket is constructed of, how well sealed it is, etc, to the temperature, dampness, etc of the burial site, many factors effect the condition.
The body of Abraham Lincoln, has been moved several times since his death in 1865. His body, however, was well embalmed and contained in a casket made of marble. When it was moved for the final time before being lowered into its current resting place (in his coffin, inside a concrete and steel tomb 10 ft. below the floor level of his mausoleum) his coffin was opened one last time to ensure that his body was still there (that had been attempted thefts of his body before). This was in 1901, some 36 years after his death. It was said that his body was still pretty much wholly intact and fully recognizable as Abraham Lincoln. Although some of his clothing and the American flag that was draped over his body had rotted away.