If you follow magazine times, in '05 Motor Trend got a 13.5 out of a stock Mustang. I have not had mine at the track yet, and I'm sure plenty of other people will chime in.
I know it's a big range, but it depends on where you live (altitude, weather, etc.), you should see anywhere from a 13.5 - 14.3.
reports have been anywhere from 13.2-14.5 for a "stock" GT. most are getting high 13s. obviously, the better driver you are, the better time you will get.
My car has run 13.71 @99.8 MPH at a track that is 1700 feet above sea level in 95+ weather. That was with drag radials( 255/60/16 ) and a SLP LM as the only mods done to the car. Track conditions and elevation as well as the quality of the driver play a factor, so times can vary due to this.