I don't know of what the "standard" answer may be but I try to give mine 4 feet between each other on the basjoos... but mine seem to run a bit more than what I expected
(sometimes pushing pups over 2 feet away).
Really it would be personal preference... get them to a distance where they look good and if they start crowding when they get bigger you could always thin the mats a bit and use the pups to start ya a new mat elswhere in the yard.
EDIT: OK one of my partners in crime just advised me that my measurement is jacked up... he said 8 feet but I still say 4 feet is cool cause I like the "oh-my-god-I-need-a-freakin-machete-to-get-through-my-yard" look.... but like I said looks will have a whole lot to do with how far apart you get them....