You will want to try to get on as soon as possible when your window opens. Many of us choose to get on right at Midnight (Eastern time, regardless of which time zone you live in) to get the best shot at it. Often times the next morning is fine, but I wouldn't wait much longer than that. Once the window opens, you'll see all the times, even the ones that are full. Only once you try to book a time will you find out if it is full. I do hope they fix that! We tried to pick times for the two M&Gs that were close to each other (like 30 minutes apart), but I have read some recent reports that they don't let you book them less than 1 hour apart. 30 minutes apart was plenty.
If you can't get spots, don't despair. Keep checking back. Often times spots will open up later. Also, additional tickets are available on the ship. When you get there, the Navigator (schedule) will tell you where to go. In a pinch, you can even try going to the event about an hour after it starts to inquire about no-shows. In our experience, by that time, they were aware if a number of people hadn't shown up and if the lines weren't long, they let other people in.
Remember too, there are LOTS of characters on the ship that don't require tickets, so you will have plenty of chances. Have fun!
