Recently purchased a home with a huge arborvitae "hedge" for privacy. Former owner planted them 40 years ago, and had them trimmed regularly until her death (house was in an estate for a couple of years, and hedges were unattended to).

I'd like to "take back" some of the yard lost to the width of the arborvitae. They're probably 20' high. The neighbor behind
also likes the hedge for privacy so I'd like to trim, but do no harm for future growth.

What's the best time to trim? How much can be safely be removed?