Length of undergrad?

I would recommend that you see what your options are. It is more important to land a good job out of undergrad than to minor or get a second major (assuming that you will work for 2-4 years before applications). Especially since many of the people that you will compete against will already have masters degrees, you minor won't look so special. So I would be doing everything you can to set yourself up for a top job. This would include landing top internships, attending networking events, starting to spread your branches into on campus clubs in your area of interest, and even gaining leadership positions in those clubs. School work will only get you so far, and the fact that you are alread a business major will be a positive (assuming you are doing well). So now it is time to start rounding out the rest of your application away from your schooling, and there are many ways to do this.

Plus, a minor might cost more $, and it might be worth while investing that money into other areas including GMAT, MBA consulting, or your MBA costs down the road. Good luck!