Depends on which island you want to go to. Key West is about 3.5 to 4 hours from Miami/South Beach.

I didn't think it took THAT long to get down to Key West from Miami....I thought it took closer to 3. I know it's an hour from Marathon to Key West (or a little over), and it's about 2 hours from Marathon Key to Miami.....but I do drive awfully fast. :D