The monorail has been operating on a varied schedule based on park hours. However, as of June 22 the monorail will operate on a fixed schedule, not dependent on park hours, to accommodate upgrades and new automation/safety features being implemented that will make the monorail more efficient in the future. This schedule will last, at a minimum, though this summer. No word on when the times might switch so just assume this is the schedule until a different announcement is made:
Magic Kingdom Resort beam - 8:30am to midnight
Magic Kingdom Express beam - 9:30am to 11:30pm
Epcot beam - 9am to 11pm

Additional watercraft and buses will be brought online when the monorail is not operating.