In our experience, it has been around 12:30 or about an hour after general boarding begins. Check the Navigator they give you in the terminal for the exact time and location. We had an 11:30 boarding time on our last cruise (on the Magic in Northern Europe), and I got to Lumiere's about 12:30. This was also the place where they were handling dining changes and Palo reservations. The longest line by far was for the beverage seminars, but I was still able to book the ones we wanted.