Most people cut their BB back around this time of year. Take it down to at least 4 feet, of not shorter. yes, it will grow like crazy in the spring...that's why it needs to be cut back! ;-) In the later spring, after it has started to grow nicely, you can prune it again to shape it a bit and/or to remove any dead branches.

Prunning depends a little on the species. Some species of butterfly bush bloom on the the previous season's wood. In these cases, if you prune them back, they will not bloom next year. These species should be prunned only after blooming if you want them to bloom.

The more common species however can be cut back to about 6" from the ground. I think cutting them back to this level makes for a much prettier plant. The stubs aren't sticking up in the middle of the bush and there aren't sideshoots sticking out sideways on the old stubs. Cutting back to this level will also mean you don't have to look at a bunch of stubs sticking up all winter and when new growth occurs, it won't be as crowded which means better air circulation.