I've seen Tokyo-HK cruises with a Shanghai stop at about 12 days. And that's with 8 total stops, two of which (Osaka/Shanghai) stop for two days.
If it was more Disney-focused (Tokyo-Shanghai-Hong Kong) they could probably shave it down to 8 days. Still long for a
Disney cruise but much more reasonable.
Shanghai-Tokyo is about two days at sea. Shanghai-Hong Kong is 2 days at sea (these are estimates based on other cruise lines, so time may vary depending on ship size I suppose).
An itinerary may look like:
Day 1 - Tokyo (Yokohama)
Day 2 - At Sea
Day 3 - At Sea
Day 4 - Shanghai
Day 5 - Shanghai
Day 6 - Cruising
Day 7 - Cruising
Day 8 - Hong Kong
Then simply run the itinerary in reverse for the next cruise. Obviously. this type of cruise would have pretty specific appeal to Disney fans. With a ship that size the bigger question would likely be could they fill it with folks who are pretty much only booking the cruise to hit up all the Asian parks in one week