Assuming you mean one of the Prunus laurels, the best time is early spring, but it can be done any time.

Agree with bamboo. If it is a small hedge try doing it by hand - hedge trimmers shred the leaves a bit, though they do recover quickly. Laurel can also be cut back right to the main stem and will still grow. If you want the laurel to flower and berry then leave it to the late winter. Some people just keep the sides pruned regularly, leaving the tops to flower and berry, then cut them down late in winter to keep the height from getting out of control.

Interesting - I didn't think anyone worried about keeping the berries on prunus, Avkg47.

Good for the wildlife, Bamboo, and looks nice - the tidy hedge has a 'hair' of flower then a 'hair' of berry, if that makes sense?! It is worth seeing to appreciate, particularly in an archway. So few people leave laurel to flower and berry - can be quite eye-catching at the back of a border or along a hedge - and I have had several people ask what the plant is, because they didn't know laurel flowers and berries!!

I can understand wanting to keep Aucuba japonica berries in place - they go that glorious red around Christmas time and are very obvious, but Prunus? They're not even that attractive, being black.

The long white flower stamens, followed by black berries, is very attractive against the green leaf, and provides an interesting back-drop to a laurel hedge, rather than just the green leaf hedge. As soon as the berries die off, or get stripped off by the birds, trim it back to height.