The answer to 'frequency of hedge' is that most hedges should be trimmed 1-2 times per year, typically in the late spring or early summer. This helps maintain the desired shape and size of the hedge, and promotes healthy new growth. Trimming too frequently can stress the plants, while not trimming often enough can lead to an overgrown, unruly appearance.
The specific frequency will depend on the type of hedge, your local climate, and the desired aesthetic. Fast-growing hedges like privet or boxwood may need trimming every 6-8 weeks during the growing season, while slower-growing varieties like yew or holly may only need trimming once a year. It's best to observe your hedge and trim when it starts to look shaggy or uneven.