As soon as you are sure there are no birds nesting in it. I chopped mine down last October (when I bought this house) from over 9ft down to about 18in height and then down to under a foot at Easter. It's now a reasonable 2 ft of new growth. Intention is to keep it like that until it comes out to make way for iron railings.

2 weeks ago OH reduced ours by 3-4 ft in height as the nesting birds have flown the nest.

Just seen Monty Don's gardeners world who said you shoud hard prune in winter.