Depending on when you go, it will likely be either 7:30 or 8 a.m. They do seem to adjust the restaurant times, just as they do the park times (please feel free not to flame if I'm wrong -- just saying it seems that way to me . ) The very, very earliest character breakfast seating time we were ever able to get was 7 a.m. at Chef Mickey's, but the 7:30 or 8 a.m. openings are much more common.
We had the first seating at 7:30 in November. Although it has hard to get going that early in the morning, I'm glad we did it. The characters came around three times and the kids got to do two parades around the restaurant. It was lots of fun!