Lines to meet Duffy can be exceptionally long. During peak times you can expect to wait several hours. Lines do tend to thin out for a while in the evening, especially when Fantasmic! is running.
Duffy wears seasonal outfits at certain times of the year, and when he does, the lines can be even longer!
ShellieMay tends to only appear in the afternoon (switching with Duffy, not in addition), and usually her line is shorter than Duffy's.