I did it two days in a row and was the only person in line the first day and the second person in line the second day.
Minnie & Pluto were in the room the first day and Minnie & Goofy were there the second day. The pictures are added to your PhotoPass which you take to the camera show near the entrance to Spaceship Earth. The CM will show you the pictures you are allowed to choose from (we had 4 taken the first time and 3 the second time).
The wait to pick out and get the picture was the worst part--probably about 10 minutes. To actually go in the room we waited no more than 3 minutes.