Varies greatly, depending on origin and destination and mode of travel and time of day, but it can vary from a 10 minute walk from the Beach Club to the Epcot International Gateway - but it is always safe to bet on an 60-90 minutes at the outside.
Buses supposedly run every 20 minutes, so if you just miss one, you could need to wait another 20 minutes for the next bus, and then the ride to the park could take 20 minutes or more, plus the time for the bus to unload and reload, especially if there are many guests onboard or if ECVs/wheelchairs need to be unloaded/loaded.