It took a little over an hour to import; I haven't checked back too far in my posts to confirm, but I'm pretty certain that all the LJ comments on my posts have been imported. Comments on the most recent posts have all been imported.

I honestly expected it to take a little longer than that, because Dreamwidth's system is dealing with so many new arrivals I got a news update that said DW had had thousands of new accounts in the last couple of days. Despite that, just an hour or so? I'd say that was a pretty impressive response time.