We saw 76 characters when we visted in March. Never timed it. Would have been interesting to see the average time spent waiting in line and the time actually spent with the character. Will have to do this on our next trip.
We tried to be considerate of the people behind us. We had our autograph stuff ready to go (used one book). Kids greeted the character together. Took the picture , character signed the book/card, and then each of the kids got to say thank you and goodbye seperately.
Got behind a few people that had what seemed like a never ending supply (pictures, books, hats, shirts, etc, etc etc ) of stuff for the character to sign. And others that had to take 20 pictures because someone wasn't looking at the camera, or smiling, or etc etc etc.
I would think 1 to 1 1/2 minutes per character (regardless of the amount of people in the group) would be fair to all of those waiting in line behind you.