It never finishes. At any given time what it says differs, but at the moment the pulldown menu says "Backing up: 2 KB of 115.3 MB" and it's not moving. It will move slowly, make some progress, then the rightmost number will creep up. It never gets ahead of whatever is mysteriously changing data on the disk, I'm guessing. Even when I leave it alone and go on vacation for a week without the laptop, with all apps closed and the laptop left running.
Eventually the Time Capsule disk (3TB) fills up, from my 1TB laptop, without ever having finished a backup.
I only ask because I'm still on Sierra and see this list of broken features under High Sierra as troubling.
Hopefully they can do a "Snow Leopard" next release - 0 new features, loads of bug fixes.