If you cut the top out that will green back up. If you cut back at the sides in many cases it won't recover and you will be left with a large area of brown stem.
I have 2 leylandii in the garden as a windbreak. They are about 15 ft high and tops get cut every few years. No idea what the tops look like - they are high enough not to be seen! Dpends very much on how high you are going to cut them.
'Optimism is the faith that leads to achievement' - Helen Keller
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In the sticks near Peterborough