It will probably have finished flowering now so i'd do it now but i think i'd reduce its height over a couple of years - i'm not too sure if reducing it by half might be too much all at once ..... let's wait to hear what someone like Bamboo says.
(I'll PM her).

Leave it till winter, when it's dormant, even though its evergreen - you can then reduce it nearly to the ground if that's what you want to do, they regenerate pretty well from old wood. You can give it a bit of a tidy up now if its finished flowering, but don't go mad this time of year.