First of all, 55% is a lot further along than you think. Considering how fleshed out the Gamescom demo was, it's obviously progressed well beyond building blocks of the game.
Second, no company uses the same percentage designator towards completion. SE's 55% is not the same as anyone else's. 60% might be their goal for CB testers. Aside from whatever percentage they're comfortable with using, closed beta often is only 65-75% of a completed game anyhow. That's why the selection process is dramatically more stringent for those who participate outside of the company's testing crew.
I'd venture that the opening days of Summer 2010 is the latest we'll see a release. Closed beta won't last more than a couple of months, and even tacking on a four-month open beta puts it in that timeframe.
FF13 has nothing whatsoever to do with FF14. Same company, two totally different target markets. Only overlapping of these two is promotional manpower/resources by the same people, which also speaks to a mid-year release. FF13 hubbub will be dying down by then.
This is what I've been saying. I've been in marketing my whole life, and followed video game marketing as well. I can tell you as a fact, the two biggest times people spend money (aside from black thursday in America) is at Christmas, and in the summer. People like spending money in the summer, because they are in a mind frame of "fun and recreation". Once school hits they concentrate more heavily on their jobs and education.
Now, no one knows for sure, we're all just guessing, but my educated guess would be a release some time in the summer of 2010. As for Beta, I am making a even more educated guess on that. DirectX11 graphics cards are said to be released anywhere from the next 2-3 months from what I'm hearing (ATI 5800 series, and Nvidia 300 series).
There's a good chance Square Enix will wait till those are released, and soon after start the beta testing so they can utilize DirectX11. Now, this is just a educated guess, but it seems plausible. One way or another, I would guess for Beta to start some time around the end of this year.
Also, as other people have said, 50% done means they have the bulk of the game designed and created. What they are doing now is simply adding things in, fixing bugs, polishing off graphics/movements/dynamics/AI etc. The next 50% will be a lot shorter than the first. Square Enix has been working on FFXIV for the last four years, so if you went by that logic, it would be another 4 years before FFXIV was released.
So, TL;DR. The best guess I would count on would be end of the year Beta, Summer 2010 Release. Edited, Sep 10th 2009 1:34am by EndlessJourney