Usually twice a year is possible with these. You can prune quite hard back into older wood in spring if you need to take them hard back. Normal pruning, say half back into last years growth just before new growth in early May and once more no later than mid July if you want to do it again. Each time you do this, you get a flush of reddish new growth of course which is an attractive feature of this plant. All I would suggest is apply a good high potash fertiliser (rose food) a few weeks before the second prune. The high potash feed helps ripen wood on new growth reducing die back during winter.

Thanks fractal, that is a very helpful answer. I shall take your advice!