Its really a creative decision, but generally you're not putting the subject behind a traditional backdrop in order to really focus on the backdrop - so ideally you'd want it to be somewhat out of focus.
So generally, the further they stand from the background, the more out of focus the background will be. Unfortunately, how far depends on too many things.
- How out of focus you want the background to be.
- How far you are from your subject.
- Aperture of your lens.
- Your backdrop
- Your lighting
Ideally your backdrop should be neutral in texture and tone enough that even a little out of focus will gently blur the whole thing and thus you won't have to get your subject a long way away.