Hi. I'm not an expert as I had the same trouble. Mine is about 15 foot tall and flowers twice a year, and I wanted to keep it shaped nicely. The guy that came round to do mine said you should do it after they have flowered and did it around the end of September last year. It's fine, has flowered this year and still looks lovely. Hope this helps!
Excuse me - can i plant my young magnolia tree on my lawn now? It's about three years old now, and was left in a pot. It hasn't flowered yet, but it seems to be growing well.