Best time to prune properly is midwinter, when the plant is dormant, or between mid November and early February. You have 2 choices - cut out some older stems down to ground level, reduce the length of the remainder by third to a half - or cut the lot to the ground. The second option means you'll lose flowers next year.
If it gets out of hand during the growing season, I usually chop of excessive growth as and when - most often after July, and its fine so long as you don't take too much.