I have found the safest time to prune a butterfly bush is in the spring after all danger of frost has past when you see new leaves emerging. Prunning it too early could kill it.

Thank you. How far back do I prune it?

I have raised many butterfly bushes. If we have a mild winter, new growth can occur all along the stem and you can wind up with a huge bush. Ususally I trim my bushes back to about 6" to 12" toward the end of April. The branches make good supports for things like peas.
Keski

I also cut mine back to about 6 - 10 inches if the trunk isn't too large for my pruners.