Well, it depends on how big you want them to be. I have a hedge which is in the 6'-7' height range right now and I will hedge prune that to about 4'. I have other knocks I will cut back to 18".

My experience is that whether they are pruned back or not, and I've done both, they will grow to about 5 feet. I've seen the landscapers prune them back hard in the early winter, and they are so tough they come right back next spring, but they do seem to stay a bit smaller then for some reason.