If you go to PortOrleans.org there is a entire Transportation section that discusses the buses.
Short synopsis:
When Do the Two Resorts Share Buses?

The ‘North Depot’, between the Alligator Bayou and Magnolia Bend areas |
The theme park buses officially start operating at 7am, although sometimes you might find buses commence a little earlier than that as they leave their home depot at about 6:30am. Many routes operate independent services for French Quarter and Riverside (especially at busier times of day) but shared services calling at both resorts generally operate as follows:
Magic Kingdom: Shared service for the first few buses very early in the morning only (for early character breakfasts)
Disney’s Hollywood Studios: Shared service for the first few buses very early in the morning only (for early character breakfasts)
Epcot: Shared service for the first few buses very early in the morning (for early character breakfasts) and also during the middle of the day, 11am—4pm
Disney’s Animal Kingdom: Shared service 11am—park closing.
Disney Springs: Shared service all day, also incorporating an Internal Resort Shuttle service for travel within Port Orleans.
At other times, the two resorts generally operate independent bus services. The switch from the initial early-morning POFQ/POR shared service back to the regular morning routing does not seem to happen at any fixed time, depending instead on factors like which parks have early Extra Magic Hours and the current level of demand. However you can expect the shared service to operate until around 7:30-8am. The last few buses late at night may operate a shared service too.