Just got very sidetracked in one of my old textbooks from forensics class... But it very helpfully says that the degradation of the DNA will depend on 'environmental factors' without giving any real statistics as to how long it would take in different soil types etc. From what I understand, if the tissue itself has completely putrefied, then the normal cellular DNA will not give any usable results, so it would depend how long she's been buried, where she was buried, if there was any factor present that may have preserved the body in part (and therefore any other tissue present on the body). However, as Drachen Jager said, DNA evidence from hair rather than skin cells will be viable a lot longer. Mitochondrial DNA has been successfully extracted from hair samples hundreds of years old. So maybe it'd be simpler to have a hair found?
I am not, however, an expert - we only had to cover a sort of basic course in Forensics for my law degree, so I'm pretty much just paraphrasing the textbook here. Hopefully one of the many experts on this board might be able to give you more useful and reliable information.