Herbs: Advice wanted re: pruning Lavenders, 4 by JoParrott

JoParrott wrote:
In my rock garden I have several beautiful Lavenders- some are volunteers. Obviously they are very happy in their homes! The first 2 photos are of the same plant- nearly a 5 foot circle-it is swallowing the boulder! The other 3 are in the same area but not as big. My questions are WHEN to prune, and HOW FAR BACK? In order to have them stay shapely do they need cutting back to about 12" or so, or will that kill them? I appreciate all help. Thanks- Jo in Richland WA