The best time to prune is after all the blooms have finished. Then give it a small dose of fertilizer for acid-loving plants (like an "azalea and camellia" formula).
But do you really want/need to prune that lovely little bush? Unless you have constraints of space, I'd leave off pruning at least for another year. If the plant gets too big over the summer, pot it up into a larger pot. It looks very happy, and I wouldn't mess with success unless you absolutely must.
