I wouldn't cut it back at all. S. officinalis grows from it's center for a long while.

You don't have to cut it back. I might try to cut it back second or third year, just to keep it down to size. Just harvest what you need for fresh use.
In my climate sage is an evergreen, like rosemary.
Now my 3 year old sage is blooming

Sage is a woody plant. So like many woodies, I would never cut more than 1/3 of the living plant material away.

I'm not sure why you want to "cut" it at all anyways. It really doesn't usually need much trimming at all. Once it gets big enough, you can harvest by pinching the terminal buds/leaf pairs to encourage bushy growth. Not much more than that is ever needed.

Once the sage gets venerable - doesn't take long as they are not long lived, it will start to get straggly. Maybe 5 years-ish. When it starts to get that way, get a new baby plant so that one is maturing while the other is ending its life. I plant a new one every 3 years or so to always keep the sage in great supply.

FataMorgana