I cut mine back very hard every couple years. I time it so its after blooming, after the first flush of spring growth, but early enough to give it time to regrow and set flower buds for next year...about mid-May.
Peter Warren says its important to keep the tree "young". This is done by constantly renewing the growing tips by pruning, both shoots and roots.
Here is an example of a fairly hard pruning on an azalea done last May. Before, about 75% of the foliage was removed, and the last photo shows it now; after winter leaf-drop, and with buds set for spring:
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