Well, the book says in relation to another variety of C. thrysiflorus:
Flowers in spring and early summer on the previous season's wood.
When to prune: midsummer, after flowering.
Training and pruning: After establishing, cut back longer flowered shoots by one-third to half after flowering. For more bushy growth, trim them again lightly later in summer. Neglected specimens are difficult to renovate, as growth does not break from older wood.