When is the best time of year to do this and how much should you cut back?

Spring, around the end of March for much of England, maybe a little later in early April for further north and Scotland. On younger plants cut back to about 18 inches, on old and woody plants cut as far back as the lowest developing buds. This why it should be left until the mid-spring so you can see the buds.

Now, in November, you should dead-head the plant reducing the height by about a third but not cutting right back.