cutting back large Leylandii - how far back

I have a large hedge Leylandii along the back of the house
Around 8' high maybe 60-100ft along the back fence
Its has over the years grown out into the garden a lot - used to be mother-in-laws property and she had it cut back every couple of years
We have now purchased the property a few years back, and have cut the Leylandii back a few times, and paid to have it cut back.

I'm now planning on getting a Stihl long reach hedge trimmer to keep it under control as I can not afford the £600 ish quotes , and would probably cut twice a year along with lots of other hedges
I'll post a question about the hedge trimmer in a separate thread

My Question re the Leylandii
How far back can it be cut - must it still have green showing , i have been told to far back and it will die

 

Thanks for that info, I will have a look at the top - I think the main trunk has been cut down and very thick, so not sure i can go much lower, its also about the same height as a fence we have which divides us from a park.
Interesting you're suggestion to cut back 3 times a year, even more reason for the Hedge trimmer - separate subject. Strap / Harness for Long reach Stihl battery hedge trimmer
any suggestions on times of year
We check for nesting, before any work.

 

Thanks,
That would require a chainsaw , which i do have , and there are about 6 trees to do along the length

What would be the advantage of fresh growth on the top ?

 

excellent thanks , may look at that as a bigger project
But may help keep the top trim
Thanks again for all the advice