I assume that you are in U.S., and want to go abroad for MBA. If you can get into schools like INSEAD, it would take a year to complete MBA. It would take minimum 2 years in other schools. If you go to international school just for the sake of going to international school, it's not going to be worth it.
If you are thinking about doing some other business course, "how long" varies depend on what degree you are going to pursue.