When you receive your Buxus Sempervirens Root Ball plant, it will need to be planted within a couple of weeks or so and must be well-watered during this time. Do not allow the soil to dry out- this will more than likely damage the roots altogether.
You should only need to trim your Box hedging once/twice a year, along with adding some well-rotted compost or mulch, to keep the hedging in its best condition. A fertiliser/root grow feed is also recommended (in Spring). You can prune back young Box plants by up to one third within their first year (this will encourage a dense, bushy new growth).
From there on, when the plant is well-established, only prune once/twice a year within May-August.