Any time it starts to look spent and straggly. Some of mine was strimmed level to the ground today by our garden friend. The others were strimmed with 3-4 inches of stem left about 3 weeks ago, just before the rain started. They are nice bushy little plants now, with a few flowers out too. The ones strimmed today may be replaced anyway Annie. As that part of the border needs sorting. I have pots of geraniums nearly on top of them. They are tough long lived plants with a long repeat flowering time if watered and fed around now.