Depends how you want it to grow - if you want it as a shrub, cut it down to 2 inches in March, or April if you're somewhere up north. It'll put on between 6 and 8 feet of growth by autumn. If you want it as a small lollipop style tree, trim back all lead shoots, particularly the top shoots (this tree has a strong apical domination habit) in March, and again once or twice during the growing season, but not later than end of June in the north, mid July in the south. Whichever option you choose must be carried out every year to keep it how you want it.

Thank you for the info much appreciated.