The Internal Resort Shuttle runs all day.
This is how it works. ANY bus will take you to the Hospitality House, as the HH is the last stop for all routes.
The Downtown Disney bus works a little differently. It starts out at the HH as an Internal Resort Shuttle... it then makes its rounds through OKW as a Downtown Disney bus, and returns back to HH to pick up for DTD.
So, the Internal Resort Shuttle runs about every 15 minutes, give or take.
Caribbean Beach is simmilar. It used to be the DTD bus, but currently the Studios handles it. The bus comes into the resort, and starts off making a complete circle as an Internal Resort Shuttle. It then switches to Studios, and makes another complete circle. This allows someone at any stop to get to any other stop within Caribbean Beach.