If they are the new Buddleja Buzz that are smaller, I think it would be in spring after the last frosts, but I'm not sure if they would need a trim even then... I have the bigger type and I lost them last year cos I cut them back after I thought the last frosts were over here and got caught out by a late frost. Luckily my neighbour had one and it self seeded in one of my tubs!
See also: http://www.thompson-morgan.com/trees-shrubs-and-ornamentals/shrubs/buddleja-buzz-collection/p90090TM

Trim back in about mid to late April, where you live. I understand the rules are similar in that you cut back to within 2 inches of old wood.