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."

Geez, I just realized I mis-spelled Forensics in the topic title.

MK,
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.

LAS