I would leave it in primary until you're at a stable SG/FG... Unless you have fruit in the mead that is. A traditional recipe (honey, water, yeast) should just sit and ferment until pretty much done. Then rack it to another vessel for a month, or two. At that time, get another SG reading and compare with the previous one(s). Rack again to get off the lees and help clear it up. You might need to rack again, depending on the honey, yeast, and other things in the mead.
Did you add nutrient to the must? Are you aerating it prior to the 1/3 break? Depending on the OG, and what else you're doing to the must, you could be looking at 3+ months before it's reached a stable SG/FG. Even then, don't assume it's ready for bottles. Let it go at least another couple/few months in bulk form. If you can let it go longer, do so. Otherwise, you'll need to stabilize the mead before bottling it up. Not a difficult task, but you need to be sure to not use either too little, or too much, of the chemicals. If you want to avoid those potential issues, let the mead go as close to a year in bulk before you look to bottle it up. You'll want to inspect the carboy's bottom every month, or so, and rack when there's a decent amount of lees on the bottom. It should become more and more clear with each racking. Once it reaches a certain clarity level, you'll just get more lees to drop out, and you'll probably not notice any visible difference in the mead.
With my mead batches, I used regional wildflower honey. While fermenting it was very cloudy. Now that it's done, the clarity, and color, is striking. Since I started the batches just before Thanksgiving 2010 I have a few more months (or more) before they're ready for drinking. I hope that at least the traditional mead is ready around Thanksgiving this year. At 18% it could be.
BTW, welcome to the boards...