This is gonna vary... depending on the subject (individual, couple or group), focal legnth of the lens, aperature.... see what I mean?
In my studio, I typically pose an individual about eight to ten feet from the background. This allows me to keep most of the light from the main from spilling onto the canvas. I shoot between f8 and f11, using a "twice normal" lens (180mm for 6x7, 105mm for 35mm).
I hope this helps.
-Pete