Love butterfly bushes! You dont have to cut them back at all. When I first got one, I decided in the
spring to cut back the 'dead' parts. It wasn't needed, although it did make the bush..bushier. The plant looks dead in the
spring, but be patient and little leaf buds start to form on even the most dead looking branches. Once it's leafed, you can trim out any really dead twigs.
I gave one to my daughter a couple years ago and it's beautiful at 6 feet! When the blooms are dead, I snip them, which usually makes side shoots appear so it stays blooming for a long time. I always snip off any remaining dead
flowers, sometimes trimming branches if they are too long. She prefers to see a trunk so I also trim off the bottom branches a bit which allows you to plant small bulbs or shade loving ground cover.