You can check the site for the times on the day of your visit:
https://www.tokyodisneyresort.jp/en/tds/greeting/
Duffy has two meet & greets. One is in his Latin American Greeting at the Greeting Dock. That will probably be like 15-20 minutes during Happy 15 and rise to 45ish within an hour of park opening.
The other is his outdoor greeting (these get cancelled in bad weather). If he appears in his sailor outfit, it'll probably be like 20 minutes at Happy 15 and then rise to 90 minutes within an hour of park opening. ShellieMay will probably be 15-20 at Happy 15 and around 60 minutes an hour after park opening.
If they have seasonal outfits, ShellieMay will be around 20 minutes at Happy 15 and will likely rise up to 90 minutes within an hour of park opening. Duffy will usually have return tickets issued with a time of day that you can return to meet him. The wait to receive a return ticket is usually like 10 minutes at Happy 15 and can take like 30 minutes within the first hour of park opening. They're usually all gone within two hours of park opening. Your ticket will have a return time written for when you can meet Duffy and a number on it. You return at the start of the session and line up according to number. In low crowd days, they won't issue the return tickets in which case Duffy will be about 30 minutes at Happy 15 and rise to 2 hours once the park opens. Depending on how quickly the line grows, they may cut off the line.