I've played on Playstation before and got the Xbox version today; my achievements took... I want to say between half an hour and an hour maybe, until I finished an in-game-achievement on one of my characters, and then they started popping up in a row. Later when I looked at the in-game achievement screen, another showed up. So it seems the Xbox network needs a moment to process it, and potentially a trigger to tell it to check for achievements. That's just pure speculation on my part, but maybe it helps someone trigger theirs.