The average time from exiting the airplane to resort drop-off is 70 minutes.
Disney's Magical Express is a shared ride for everybody. There's no such thing as a resort that has a DME bus all to itself ... unless there happen to be 50 people going to the same resort at the same time.
Each bus goes to no more than 4 resorts. There are no pre-set groupings of resorts, and no pre-set routes, so there's no way to know in advance. The dispatchers decide what resorts get grouped together on any particular bus. But the driver decides on the order of drop-offs. Since Pop Century is pretty much the closest resort to the airport, most drivers with Pop guests choose to drop-off Pop first ... but again, that's not a rule.
How long you wait, if you wait at all, depends on when you get there. If a bus just left, you'll wait a little longer. If a bus is just about to leave, you won't wait at all. There is a rule/procedure that a bus must leave within a certain amount of time after the first guest boards the bus. In other words, they aren't supposed to wait for the bus to fill up. The last time I took DME from the airport, there were 12 of us on the bus.