I generally give Buddleias a hard prune in early spring to keep them shapely when new growth starts to sprout all over. I cut back to an obvious point of growth but frankly they will grow back from wherever they are cut to. Buddleias are fairly irrepressible including in spots where you might not want a Buddleia, like growing out of a drainpipe halfway up the house. But so lovely in places where you want masses of nectar-rich conical flowerheads attracting butterflies and bumblebees in high summer.