in the studio....

When taking studio shots, how many feet do you keep your model in front of background?

What different effects can you get?

Because of my limited space I can get about 4 feet from subject to background. I can push things a bit if I back the camera up all the way and then I get get about 6 feet between subject and background.

Depends really... If I am limited on space I'll grab the 50 and go to 1.8 or 2 aperture to blur the BG.

If not, I just put more distance between model and BG.

Unless I am doing just a portrait (in which case it is 5-6 feet) my models get a box to play in that is 3 to 7 feet from the backdrop, although I try to center them on the 5 foot mark from the backdrop (which is were my lights are set). It is just easier that way.
I would have them 8-10 feet, but I just don't have enough room for that.

With at least 4 feet away from backdrop I can make the shadows go away on a white background and that is all that is important to me as far as separation is concerned for my plain studio backdrops.

Just my experience,