I checked a bit further...
"Professional Microphone Techniques" by Huber & Williams
Spaced Onidirectional Microphones
Common spacings are from 2-10 feet on either side of the centerline. The spacing is determined by the width of the sound source and by the mic pair's distance from that source. A general rule is that each mic should be placed one-third to one-half the distance from the centerline to the outer edge of the sound stage.
So Nika...seems like there really is no specific distance...but I'm sure you've have some prefered "numbers" based on your hands-on experience with this type of mic setup.
I don't really use it.