In Georgia, the best time to prune butterfly bushes (Buddleia davidii) is either late winter or early spring.  At that time cut them back to a height of a foot or two above the ground.  This will help promote new growth and a bounty of blossoms later in the year. 

       The average life expectancy of a butterfly bush is 10-20 years.  Consequently, once older bushes begin producing fewer flowers, it is time to consider replacing them.