The spacing depends on the size of the room and the distance of the 'back' speakers to each other.
Say if the 'back' speakers are 10 feet from each other, the surround speakers should be about 3 feet from the 'back' speakers, the 2 surround speakers would then be 3.5ft from each other, sitting behind the viewer.

If the surrounds are the same speakers as the back speakers (like they should), you try to keep the distance from the viewer the same, so the 'backs' say are 5 feet from the viewer so the surrounds would be pushed back that too, so the sound level will be even all around. Or if room behind the viewers is limited you would have to go into the receivers speakers settings and drop the db of the surrounds to balance them with the 'back' speakers.

So you have two things going on X and Y axis; spacing distance from 'backs' speakers too the surrounds speakers and 2nd distance from the speaker (in this case surrounds) too the viewer.