In your climate, box can be pruned just about any time you want, although I'd be inclined to hold off until late Feb/early March. And you can prune it back quite hard, although it may take some time before it recovers a decent appearance. Thinning is really the most important aspect of pruning box - it is very dense and the exterior growth shades out the interior. And if you are hedging it, keep the top narrower than the base - sort of a cut-off A-line. This will allow sufficient sunlight to reach the base and keep the plant from developing 'bare legs'. Most hedges require at least annual pruning to keep them tidy and looking good.

Here is a link that might be useful: boxwood pruning link

I'll second waiting until late Feb/early March if you're going to shear it - then you're pruning right before a new flush of growth which will cover up that just sheared look. But do that thinning any time. If a consistently good appearance is really important, I would thin first so light gets to the inside, then do a light shearing to keep the outside tidy until new inside growth occurs. After about two years then you can do a harder pruning of the outside. Sometimes you get lucky and the back side of the hedge isn't visible to anyone other than the gardener, in which case I'll thin, hard prune the back, and lightly prune the front while I wait for new inner growth.

Thank you both for the excellent advice.

don't forget deadwooding and suckering! It will be easier to get at if your cutting back hard. you will be giving it lots more growing room .lower branches on buxus have a tendency to "root in" Even though its a shrub or hedge trying to keep at least a couple of inches of clear clean stem may take a few extra minutes now but save you hours of crawling and cursing when you have to clean under it!

As a follow up, the hedge has come back beautifully. It is a small hedge surrounding a rose garden, well smaller now. thanks again for the advice.