A couple of articles for you.
Persistence of spermatozoa and prostatic acid phosphatase in specimens from deceased individuals during varied postmortem intervals. - PubMed - NCBI
The recovery of seminal components and DNA from the vagina of a homicide victim 34 days postmortem. - PubMed - NCBI
The second is a case study from Maine where they were able to recover both seminal components and DNA from a female 34 days postmortem.
and lastly this one:
[The forensic expertise of cadavers of females suspected to be victims of sexually motivated homicide]. - PubMed - NCBI
It's why DNA has become an intriguing field. Even with touch DNA there is conflict on how long that DNA lasts. Let's say you're being tried for a murder, your DNA was found at the crime scene. You could argue, and win, that your DNA was left there long before the crime occurred, possibly even years earlier, because there is no "set" time for the degradation of DNA. In a corpse, on the soil, under corn leaves, there is a difference in the rate of degradation of the corpse itself, but interior areas, for lack of a better term, hold on to things like seminal and epithelial DNA. The putrification process makes "pockets" for preservation of DNA, much like teeth (in older cadavers) and the inner ear (in less degraded cadavers).