The time it takes for the bus to arrive depends how well the airport is made, since the bus will spawn from a parking of the "vehicle" type and will try to take paths of the "vehicle" kind (rightly so), even if they are longer.
If the airport AFCAD is well made, with vehicle parking reasonably close to the terminal they are support to serve (as in real life), it shouldn't take very long for the bus to arrive. If they are too far away, or even missing, GSX will use any parking it can, so the time for vehicles to arrive can change a lot.
Of course, you can always use Time Acceleration to speed up things.
