Pruning Knockout roses is usually quick and easy. After all, they are bred to be virtually maintenance free. Isn't that why you plant and grow them?
According to Conard-Pyle, these actually don't require the pruning. You can do a cutback prior to spring blooms in the early spring. You can cut off about half of the current plant back. Conard says that you may be able to leave only about 12 inches of cane left. That seems a little low to me, but you should be OK as long as the last frost has passed for the season.
After you first plant them though, its generally best to let them grow through uncut for a season. This will help them grow unchecked into a natural position and shape. Even though they are marketed heavily as being care free, some basic housekeeping will keep them packed with blooms and healthy! Aside from the springtime cutting, you should also always cut out any dead or diseased wood to protect the health of the rest of the plant.
While these plants are designed so that the spent petals and blooms fall away, some deadheading can help out and speed up the process. The result should be even more blooms, and quicker! Normally it might take four to six weeks but with the knockouts it tends to be quicker. Some Miracle Gro can help as well.