It's after the all the flowering has finished :o) around May or June.

It is suggested in February 2012 RHS mag Garden that, to renovate old overgrown plants, hard prune some of the branches in Early Spring to within a few centimetres of the main trunk. It is suggested to stagger the pruning to avoid stress. I pruned a mature one at the base as it was shading a small lawn. This was last year after flowering. We had very little rain after for weeks. It leafed up well at the bottom but no flowers. The top half is fantastic. If you do prune hard make sure you can provide enough water....otherwise just a trim.