I have a Rose of Sharon planted from a whip about 8 years ago. It has
been a steady producer of blossoms. I have been pruning it down to keep
it from covering

I prune mine back when the seed pods are forming (September), and I
cut the hedge back about 1/3. This encourages a thicker hedge the
following year. I have another ROS hedge that hides the compost
bins--I have not trimmed back this one and it is over 15 feet tall (it
must be happy next to all that compost.) These grow like weeds and
you can probably trim them at any time and often.