I've a friend who grows one as as a bush, she says it grows swiftly and she has to prune to keep it in bounds. That one is over 6' tall and about as wide. It would be denser but she often cuts out lateral branching. Neat foliage plant.
In regards to your query: depends on how often you're willing to prune to keep it as a hedge. The closer they are together, the greater amount of pruning but the denser the hedge. If I was to chose spacing, I'd try 3' or 4' apart.