Best done after flowering next year now. Pruning is not usually carried out on these at all, but sometimes its necessary for various reasons. If you want to reduce it a lot, best done over 2 years, both times just after its flowered or by June.

Some of the lankier varieties can benefit from occasional heading back. As Bamboo said, prune right after bloom, and cut back no more than 1/3 of the longer, stringier branches. The trick is to cut just above where one year's growth ended, and the next year's began--that will encourage several new shoots to develop, while random cutting just results in one or two sprouts. You may need to go in later to rub off shoots heading through the center of the bush.