Prune Knockout roses early each spring until the buds begin swell in spring. The first step is removing any dead or winter-damaged canes by cutting them off close to the base of the plant. Prune the remaining canes up to 50 percent, maintaining the desired shape and size of the shrub as you prune
Late summer and fall are the worst times these plants should be pruned, as fall pruning can lead to winter kill.
are known to be very hardy in most regions. It is generally a repeat bloomer, likes to bloom in the spring, is disease- and pest-resistant and drought-tolerant. ... However, severe frost can seriously damage or even kill unprotected knockout roses.
Prune Double Knock Out roses by taking roses for your home any time, but give the plant at least two full seasons before major annual pruning. ... Pruning stimulates new growth and leaves the plant vulnerable to frost damage if you prune too early in the spring. During the growing season, shape up errant stems as necessary.
It's common for Knock Out roses to stop blooming profusely sometime between their third and fifth years if they don't get enough sun, water or fertilizer.
And this instruction comes from the FAQ SECTION OF KNOCKOUTROSES.COM——
“In the spring, after the last threat of frost has passed, 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.”
And another extension article recommends this pruning—-
Cut the canes back every year in 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.
So, there is a range of 12-24 inches for how far back you should prune. You might want to prune half way between--18"!