It depends upon time of day, park, where you are trying to get the autograph, etc. At some of the places where the characters are frequently out (like at the Toontown Hall of Fame at MK or in the Camp Minnie Mickey area at AK), you can from time to time find relatively short lines for the characters. When the characters come out after a show or something like that, this is harder to do.
One place that generally does seem to have short to no lines is by the International Gateway at Epcot. Often in the afternoons, some of the Princesses and certain other characters will come out and the lines are usually not very long at all.