I would say by the weekend of the 11th (is that three weeks before?) or not at all.

You've got plenty of time. For at least 2, counting the one you did today. Personally, I like my last long run a couple weeks out (or sometimes even less). I just take that run pretty easy - and generally make it 18 or so. I've done long and really long runs just 6/7 days before a marathon without a problem, as long as they're easy. So if you want, you could do one (or two) more like today's. But I don't really think you have to worry about it, to tell the truth. Your training has been so consistent and your mileage buildup so steady, that I wouldn't worry about the long runs. (Yeah. I know. People like to argue the point.) If it were me, I'd maybe do one more 18 miler, with a little MP mileage at the end. And focus more on your tempo and MP runs (which it looks you're already doing), maybe do a couple mile interval workouts (8 x 1 at slightly faster than MP, or something similar). Shorter me: run long if you want. But if you don't get the chance, I wouldn't worry about it. You're ready. Just sharpen the blade. The sharpening is where I'd focus now.