Fall trimming of knockout roses
You can remove any dead canes now or anytime but the prime time to prune your Knockout roses is in late winter or early spring. Since they are preparing for the winter season pruning now could stimulate new growth which would make the plant susceptible to frost damage.
The amount that you want to prune is fine. The following websites suggest the amount to prune which gives you the range you are looking to prune.
“Since KnockOut® roses only reach a maximum height of 4 feet and a width of about the same they do not require much pruning. Simply cut the canes back every year in late winter or early spring to approximately 24 inches. Cutting them back very severely, like a hybrid tea rose is not recommended. They won’t die but will require a long time to recover. Periodically, older, declining, and broken canes will need to be removed. This can be done at any time of the year.”
And from “knockoutroses.com”, suggests the following:
“In the spring, after the last threat of frost has passes, cut back your roses (using hedge loppers) to about 12”. This should make them grow to about 3’ – 4’ tall by the end of the season.”
I hope this information helps you out and if you have any further questions don't hesitate to contact AaE again.