There isn't a set schedule. The resort staff will explain where the bus stops are located (some resorts have more than one). As the PP noted, busses are dispatched about every 20 minutes; more frequently during high seasons. If you just miss a bus and see it pulling away from your stop, you'll probably have about a 20 minute wait for the next bus to that park. Sometimes, you'll walk up to a stop, your bus is there, you get on and you'll be one of only a few passengers. They do NOT wait to fill a bus or anything like that. There are always outliers where a wait might be 30 minutes, but that's been very rare in my experience. We time our waits and typically, we wait 7-10 minutes.
For Magical Express, there is a schedule and you need to arrive for your scheduled bus.