If you're wanting it to be blooming and looking like a plant during winter, then trim down as much as you need to for that scenario .. If you're looking to have it go somewhat dormant then go shorter. But whatever you do, always root cuttings for insurance when you cut back a plant in a pot.
For all the plants I cut down like that for winter, I trim the roots about as much as I trim the top part .. as in, cut the plant down by a half then trim the roots by a half etc.
