You can reduce it anytime, but since it's an annual, its season will end in a few weeks and it will naturally die back. If you like, let a few flowers go to seed, then save the seeds for next spring - free plants!

But if its the perennial one I'd wait until it finishes flowering before trimming it.Unfortunately you didn't fill in your county on your profile so its difficult to know whether it will he hardy enough to survive the winter.
If you leave reducing it until spring the flowered stems will help to protect the crown over the winter.

Also if it is a perennial one you might want to wait until spring [certainly in the uk]. then any winter damage will be removed. you could trim some back and use non flowering stems as cuttings so you have more for next year.

I live in Cornwall so we do tend to have mild winters. It is in fact three years old and gets bigger with more flowers every year so I suppose I am lucky. Thanks for the advice- I will leave it until Spring.