As
DCL Repeaters mentioned, Magic and Wonder both did 3/4 night cruises the first year. The other thing we have to remember is we are now talking 6 ships in the fleet AND a new Disney port. Companies do not always follow historical patterns. They follow the best financial patterns and modeling for the business.
Disney focuses on those 3/4 night cruises as that's where the biggest profit is for them. They're the most popular with new cruisers and by adding a new ship or destination to it they attract seasoned cruisers as well.
Its hard to say exactly what will happen. It most certainly possible for the Treasure to go to the longer 7 night sailings, financially I could see it being more profitable for them to put it on a 3/4 night for the first year. I really hope its going to 5+ night sailings as my family is wanting to focus on longer sailings but time will tell!