The ideal time to prune a broadleaved evergreen shrub - like boxwood - is in spring as it is pushing new growth. Your can also prune into the summer as well, although pruning tends to stimulates additional growth so a hot, dry summer can create unnecessary stress on a freshly sheared plant.

Typically, one wants to start the training into a neat hedge shape early in the life of the hedge so you do not need to remove much growth at any one time. Keep the base of the plant slightly broader than the top so that sunlight can reach evenly to the base.

Here is a link that might be useful: PlantAmnesty - pruning hedges.

Hi, I ended up here researching about making a boxwood hedge, I am wondering if you could give us an update on how yours is looking, I have a spot where I might be using winter gems as well, so I am very interested :) Thanks!