We have about 50 meters (165') of cedar hedge between our row of townhouses, and the row behind us.
Depending on the trees around it, it is between 10-12' in height, approximately.
I think they are white cedar, aka arborvitae; maybe a specie of Thuja or Leyland cypress.
Our problem with those hedges is that they are getting more and more sparse; we see more and more through it, every year.
We had very different opinions on what we should do with them, to help them grow thicker. Should we cut their head, maybe to something like 8-9', and give them enough earth+compost+fertilizer? We were told by some people that those kind of trees should not have been left to grow this tall, but that if we cut their head too much, they're going to die. Others are saying it's fine to cut their head as much as 3-4', as long as it's done in the autumn or spring.