I'm not sure if you've found this information elsewhere, but this is what I've been successful with:
If you live in an area where it's a perennial and grew pretty big: Trim it down as far as you want, up to maybe 12" down the stem (or longer, if you wish and your plant is 4 feet or more in height). Trim it right above a leaf pairing, as that will help the plant regrow, usually with a new set of twinned branches.
If you live in an area where it's an annual: Trim it down as far as you want, but make sure to leave 3-4" of stem on the plant; the 3-4" (or more) above the ground needs to be paid attention to, since an annual plant won't grow anywhere near the heights of a perennial one. Also make sure to trim it right above a leaf pairing, as well, as that will help your plant become bushier (which is good, because bushier means more stems to trim in the plant's short life). :)
I'm not sure if you've found this information elsewhere, but this is what I've been successful with:
If you live in an area where it's a perennial and grew pretty big: Trim it down as far as you want, up to maybe 12" down the stem (or longer, if you wish and your plant is 4 feet or more in height). Trim it right above a leaf pairing, as that will help the plant regrow, usually with a new set of twinned branches.
If you live in an area where it's an annual: Trim it down as far as you want, but make sure to leave 3-4" of stem on the plant; the 3-4" (or more) above the ground needs to be paid attention to, since an annual plant won't grow anywhere near the heights of a perennial one. Also make sure to trim it right above a leaf pairing, as well, as that will help your plant become bushier (which is good, because bushier means more stems to trim in the plant's short life). :)
I hope this helps!