I prune mine hard about mid February.

If you don't prune the whole plant, at least prune away the suckers and excessive lower growth that saps the plant but produce few if any flowers. I prune away anything below two feet from the ground.

I had read to prune it to 12 inches right after flowering but that contradicts the February statement above. Now I'm confused! FTR mine is so new I've never pruned it but will need to this year.

I prune all mine to 12" or so. Don't know about suckers because there's so many branches. My oldest butterfly bush is 9 - when we built the house in 2000.