I prune my well established Buddleia(s) twice.
Once in the autumn just after flowering. I use this main prune to reduce the overall height and give an overall shape. Also I cut down to two buds, one bud and a spare.
(I keep the thicker stems for supports for next year)
Then in the spring, once life is showing, a much lighter prune. Because of the autumn prune, some of the bigger branches/stems sprout like crazy. I cut out any dead from over winter and again I cut down to two buds when more sprouting has occurred.