It depends on whether you are trying to get photos with some of the absolutely most popular characters. For example, if you want to get a photo with the 7 dwarves, assuming they are out this year, then you probably need to get in line by about 6 unless you want to spend the whole party in line for them. Belle and the Beast in their holiday clothes is another one that you would want to be in line for a decent time before the party if they are the most important, as that line also is one that gets huge fast. If either of those sets of characters are a priority for you, then your ADR is too late. If meeting the most popular characters isn't a big deal for you, then you are probably fine.
To give you an idea, we went to
MVMCP multiple times last year. On a non-sold out night, we got in line for Belle and Beast at about 6:15. We were no where near the front of the line. I would say we were between 10 and 20 groups back. It took about 15-20 minutes once the party started for us to see them.
Cinderella was meeting with the Prince, Stepmother and Stepsisters last year which was a huge priority for DD to get a picture with all of them together. Again, on a non-sold out night, I got in line for them at 6:00 p.m. because they were DD's No 1 choice. I was first in line, with another lady right behind me. By 6:30, the line was probably 10 to 15 groups deep.
We actually have never done the Dwarves because DD isn't interested enough in a picture with them to put in the time necessary, and she always has a princess she would rather prioritize, but I know that some people actually start lining up for them about 5:30, the time of your ADR.