If you're looking for a schedule like a city transit system has, the isn't one. Disney buses run about every 20 minutes.
The first bus to arrive at a Disney resort will be at about 6:30a, unless a Park opens at 7a or earlier, in which case the first bus will be an hour before opening.
After park close, buses will run for at least 90 minutes, but longer is the are people still needing to get ti their resort. Disney wil not leave you stranded at a Park.
Steve