Avoid March-August is the RSPB advice. See link

https://www.rspb.org.uk/birds-and-wildlife/advice/gardening-for-wildlife/plants-for-wildlife/garden-hedges/hedge-law/

Cheers John

I'm no gardener but in my humble opinion, late winter once all the berries (assuming it's berry bearing) have gone and before nest building starts.
Without looking it up, if you suspect any birds are nesting in the hedge it's probably illegal to cut it back