MGM is really quick, maybe 5-10 minutes... once you get on the bus and leave the resort. MK is farther, maybe 15-20 minutes. You also need to be out there in time to wait for the bus (in case one is not there waiting for you). Sometimes I see them just pulling away as I round the corner. What time you need to leave depends on whether it's EE day or not. EE days for MK are Sundays and Thursdays, MGM is Tuesday and Saturday... if I remember right.