There are busses that run from park to park. When you leave the Animal Kingdom, there are signs that will direct you to the bus area (you go to the right when the walkway splits). A little further up there is a sign that will tell you what # the bus to the MK is. Once you get to the bus area, you'll see where each "waiting corral" is numbered.
We took the bus from the MK to AK last week and I'd say it took 10 minutes tops.