We were there in September, and they had a line forming.
These are characters in demand, I'll tell ya'!
We arrived in MGM on a Saturday, about 10:00. Seeing Lightning and Mater were high on our priority list, so we asked where to go see them and headed on over. The CMs had the lines cut off and we were told the Cars were leaving but would be back at 12:30.
We came back at around 12:40, and the line was already being cut off by the CMs. They didn't even stay out for 30 minutes (they were driving back by 10 till 1), and I guess we really needed to have been there much earlier than even the time they're supposed to come out.
After that, they didn't come out until the parade, and then there was one more showing, which we weren't in the park anymore. So on a Saturday in September, there were three opportunities to get your pic with them, with them only being out for around 20 minutes or so. Now, it was DEATHLY hot. Maybe in the cooler months, they stay out a bit more.
So get there early if this is a priority for you!!! I wish we had. Now we know for our January trip. Hopefully Remy from Ratatouille won't be as difficult to say hello to!!!