There really is no set distance, it takes experimenting. You normally don't want the driver on the front of the speaker to be the same distance from the side wall as it is from the wall behind the speaker. Obviously, you would not want it shoved right up against the wall either, would look stupid.
You also want to allow room behind the speaker for the rear ports to breath, maybe 4-6" or more. Also keep in mind to big of a screen can be distracting, it depends on how far your going to sit back that determines the screen size and/or projector.
