Usually only pruned to keep a shape (to remove any straggly branches or any dead wood).
If you must prune then do so as soon as the flowers have dropped - new flower buds start to form over the summer so pruning from summer to flowering will seriously reduce the next season's flowering.

Don't prune it at all unless you really need to.

Agree with both above, plus, for extra bushiness, make the cuts just above where one season's growth ended, identifiable by the rings of crowded bud scale scars circling the twigs.