Gardening 'Expert' needed.

[conny]

I need someone with a knowledge of hedges. I have, what I believe, a Mountain Box hedge, also possibly known as Kinnikinnik according to my phone app.
Although I trim it with a hedge cutter a few times each season it has ballooned in depth to about 4' and a height of 5'
I want to reduce the depth of it to about 2' but cannot find anything about it either in my books, or on Google websites. Does anyone know when I could prune it right back, (I'm assuming autumn but it is an evergreen), and how severe I could prune it without harming/killing it?
Not the best image but hope it helps.

 

[EddieM]

No expert here, but I don't think box responds well to heavy pruning.

 

[conny]

That's what I am afraid of. If it was my own I'd go for it but as I'm in rented property I'm quite dubious in case I kill it and have to replace it.