DA in England say to prune just once a year, and to take off about a third. That said, we do find that some benefit from a harder pruning than others. We have a hedge of Harlow Carr, which fell over two years ago because we hadn't pruned it hard enough. Cutting to to half its then size worked wonders and since then it has behaved perfectly. We find that some varieties, like Lady of Shalott (gorgeous in case you don't know it), Claire Austin (also wonderful) are vigorous and need quite a tough pruning, to around half; others like Kew Gardens, Graham Thomas, Winchester Cathedral, are happy with a gentler approach. DAs grown as climbers (for us including the Generous Gardener, Crown Princess Margareta, Albrighton Rambler, Fighting Temeraire) we let grow to the height we want before pruning. The good news is that they are very tough and if you get it wrong (as we did with the Harlow Carr hedge) they forgive you and bounce back!