Hedges need to be cut back once every year. This goes for Leylandii hedges as well as Laurel, Buxus, Yew and Privet.
If you maintain your hedge at a manageable height, it doesn't take very long with a decent hedge trimmer. If you consider how many times you mow your lawn in a year, cutting a hedge is far less work and it is far simpler than having to paint a fence that could fall down after a few years.
Leylandii hedges should be trimmed once or twice a year.
Some people advocate trimming Leylandii three times (or more!) every year, but this is unnecessary and, can weaken a Leylandii hedge during years of drought or other times of stress.
Leylandii hedges that have been cut too regularly do not get sufficient chance to develop re-growth before springtime, so they are more inclined to suffer from problems.
If you don’t trim your Leylandii hedge every year, it can grow up to 3ft within a year and rapidly grow to a height where you will require the assistance of a professional tree surgeon.
If you maintain your hedge between 6 - 8ft (2-3m) it is much easier and cost effective to manage than if it is 10 -15ft (4-5m) tall.

TreeworX tree surgeons trim the majority of our client’s hedges once every year.
We also maintain tall hedges that require mobile elevated work platforms ( MEWP) and we use various platforms and work methods to reach these hedges.
Leylandii hedges can provide excellent windbreaks for properties and also provide excellent privacy and screening.

TreeworX always leave about 4-6 inches of green foliage on the hedge, before getting to the brown dead foliage inside the hedge.
If the hedge is cut back very hard so there is scarcely any green foliage left, you can encounter problems if the hedge undergoes any type of stress, such as excessive heat and drought that can burn the foliage or decrease the hedges capacity for regrowth.
Your Leylandii hedge should be trimmed back to the same height and width each year, this ensures your hedge will stay the same height and width and won’t grow taller or wider year on year.
Hedges should be left slightly wider at the base than at the top.
Hedges should be trimmed in the spring or summer months. This will give your hedge a chance to recover and enjoy some re-growth before winter.
Hedges should not be trimmed during periods of very hot or dry weather.
Recent studies have shown that having your Leylandii hedge cut in late autumn can make it more vulnerable to winter attack from the Cypress Aphid.
If your hedge used as boundary with a between neighbours, they should be liaised with as their side will also need trimming.