I've tried all three of the methods previously mentioned to concentrate cider. So, here's my two cents.
Boiling requires the least effort and the cider concentrates easily. One time I tried concentrating till it had a syrup consistency. When it cooled, it was so thick that I couldn't even pour it. I've used concentrated boiled cider to make cider a couple times. I used pectic enzyme, but no finings. The cider was a bit cloudy, but nothing too excessive. My issue was with the cooked apple flavor. I doubt I'll ever do it again.
Last year, I tried heating to about 180 F. There were wisps of steam coming off, but not near boiling. I don't recall exactly, but I think it took about 4 hours to reduce it by a little less than half. It's still in a carboy, so I'm not sure of the results. It seemed to change the flavor a little, but nowhere near the boiled cider.
I've found freezing to be a much better method. I posted the basics of my method in the thread that GF mentioned. Concentrating gets harder as the gravity goes up. Getting up to 1.090 is pretty easy. I've gotten my cider up to 1.140 (32 Brix) for ice cider without too much trouble. A commercial ice cider maker has to look at labor costs. They probably freeze a cider once and they only save 1/4 to 1/3 of it. The rest gets dumped. My labor is cheap, so I refreeze the middle third of my melted cider. The cider water that I dump typically has a gravity of 1.006 or less.