You'll need to tell us what hedges you have ( shrub type) but generally late spring or early autumn are good times for most hedges.

As Anchorman says, it does depend on the type of hedge but I think autumn is best as long as you are not cutting off berries which the birds need so badly in winter. In late spring the birds might be nesting in your hedge, so do take care!

Evergreens, including conifers, can wait 'til late spring (minding the birds, of course), and I do my beech hedge in late summer. Yew, for a hard cut, over winter sometime, but generally in summer. Phil J