Buses start running to the parks about 6:15-6:30 am for those very early morning ADRs in the park restaurants. They run about every 20 mins or so. Some resorts have mulitple stops (POR, POFQ, CBR, CSR) while others share with other resorts (BWV/S/D, sometimes YC/BC). POP and AKL are the only resorts that have the one stop, no other resorts on the route.
Depending on where the resort is, and how many stops it has to make, it could take up to 35 mins to get to a particular park.....it varies quite a bit.