The migration time seems to be largely dependent on how much disk space you're actually using. In London, I'm getting a 43.75 MB/s rate on my primary disk. It's using some kind of compression though, as an empty disk copied across instantly; it's been about 10 minutes for a 10gig drive.

Edit: Ah, just misread the console. It's still "copying" my empty drives, at the advertised rate. So dividing your disks by the 40ish MBps is a good estimate of migration time.