From FORENSICS: A GUIDE FOR WRITERS, by D.P. Lyle, M.D., pg 201:
"In living victims, the duration of sperm motility is from four to six hours. If motile sperm are found in vaginal swabs, the sexual act likely occurred less than six hours earlier. After that, the sperm die and begin to break down and fragment, and timing becomes a guessing game. First the tails are lost, leaving behind sperm heads, and then the heads and tails undergo fragmentation and destruction. The survival of sperm heads and sperm remnants in various body orifices is extremely variable, so that no accurate timeline can be established. In general, these remnants may remain in the vagina for up to six or seven days, the rectum for two to three days, and the mouth less than twenty-four hours. In cases of rape-homicide, sperm may remain in the vagina of the corpse for up to two weeks.
Elevated SAP levels can be found in the vagina for up to seventy-two hours. Semen is a fairly resilient substance. Laundering and dry cleaning of stained clothing may or may not remove all traces of PSA and SAP, and traces of spermatozoa can sometimes be found on microscopic examination of the laundered material. If protected from extremes of temperature, harsh chemicals, and other unfavorable environmental conditions, dried semen stains may remain identifiable and usable for DNA analysis for many years."
Thanks, John -- I actually thought to write to D.P. after I posted here, and got a response from him today. Here's the text of what he sent, for anybody who's curious about this subject:
"This determination is always a best guess and the ME would not be able to narrow it down to a half hour window. But, he can make a broader estimate.
At the crime lab a search for spermatozoa in the vaginal, oral, or anal samples would be done. In living victims, motile sperm may be seen up to about 12 hours after intercourse and rarely up to 24 hours. Non-motile sperm may persist for 2 or 3 days. As sperm die off, they initially lose their tails, leaving behind only sperm heads. These may be seen up to 7 days after intercourse. So if the victim states that she last had consensual intercourse three days earlier, the finding of non-motile sperm or sperm heads is of little help, but any motile sperm could not have come from that episode.
Sperm survive longer in a corpse than in a living victim because in the living the vagina produces certain chemicals that destroy sperm, while in a corpse sperm are destroyed only through decomposition, which requires many days. Motile sperm might persist for a couple of days and non-motile sperm and sperm remnants can sometimes be found for up the two weeks in a corpse.
So, if your ME found a fair amount of motile sperm and had determined that the time of death was approximately 24 hours earlier, he might conclude that she had had intercourse around the time of death. If she lived more than a few hours he would not expect to find many motile sperm--every hour that she is still alive after sex more and more sperm would be destroyed. The closer to the intercourse she died the more motile sperm there would be. His experience would come into play here and he would make his best guess. That’s the best he could do."
While there are many pros here, for forensics questions, I also recommend you check out the Yahoo group Crime Scene Writers. Experts like Dr. Doug Lyle answer questions such as yours in detail all the time. If you join, you'll find the archives a good place to start.