Most of the buses are on the road by 7:00 a.m. There is no published schedule because the frequency of bus movements changes through the day based on requirements. As a general rule you can expect a bus for your destination to arrive at your bus stop within 20 minutes, usually a lot less than that.
In the evening the bus system shuts down in stages, based on when the parks close. Generally, if you are in a park at closing time you can expect buses to run from that park to your resort for another hour or a bit more. Finally, after all the parks close the buses will continue to run from the resorts to Downtown Disney and Pleasure Island until 2:00 a.m.
The resort monorail is also running by 7:00 a.m. It shuts down about an hour after MK or Epcot closes, whichever is later.
