I would wait until October, if pruned now it will start new growth it cannot support.

Should be done immediately after flowering, so don't take too much off this year - prune it back a bit and then hard prune next year at the right time.
Just seen Docbob's response - that's why I said don't hard prune now, but it will be okay to prune back a bit, this shrub is often used as hedging and would be cut in early august about 4 inches all over anyway.

I've just cut mine back this very day, by about a third, because I was advised to prune while the leaves are green rather than red. I will let you know how it responds!

Depends on how quickly winter sets in, Sheilabub - cutting back hard now might mean it puts out lots of new growth which doesn't have time to harden off before the weather cools significantly - if we have an indian summer and a mild October, it'll be okay, but if September is cold and October worse... that's why the advice is not to go too mad with pruning from now onwards.