It depends, what environment are the remains in? Is the body buried or exposed to the elements? If the body was buried in a "sealing" casket and if that casket was then placed into a "sealing" vault, the skeleton may remain for several years. All soft tissue is fully decomposed in a couple of months. If however, the body was buried in a non sealing casket and placed in a non sealing vault, then bone decomposition will take place much sooner. All of this of course also depends upon if the body was embalmed and if the embalming was done properly and sufficiently.
Human remains that are left exposed to the elements will decompose much faster due in part to microorganism activity that degrades the structures. A common misconception is that the bones will remain intact indefinitely, this is simply false information.
depends on tghe environment
for example someone who died in water, bones would not deteriorate at the same rate as someone who died in the north pole or a dessert