We had a 10 am appointment for both the Frozen and Princess meet and greets. Both times we showed up on time, yet had to stand in line past our appointed time. Frozen took us 15-20 minutes to reach Anna (no other characters were there). The Princess m&g took 10-15 minutes to get to the first princess. From there, I felt like they were rushing everyone through. A cast member grabbed your autograph book for the character to sign. Then rushed you into a picture pose, snapped two pictures, and then started trying to usher you away. Even the non-ticketed character experiences were like this though (except with even longer lines... one line we waited 45 minutes for Tinkerbell, and that includes lining up 30 minutes early (had we lined up closer to the start time, we would have been there for even longer as the line wrapped all the way around the 4th floor lobby area).
I'd suggest trying to get to the M&G a little early and if there's a line see if they'll let you get in it a few minutes before your designated start time.