I think the problem here isn't how much time you have known this person, I think the problem is that academic recommendations are more or less worthless when applying for an MBA. Several schools specifically say to not use academic recommders, while other schools just highly discourage it.
You didn't say where you are applying to, but almost all schools ask for professional recommendations. I think you'd be doing yourself a favor by following their advice.
Thank you guys for your answers.
Actually I am applying to a PhD (but that forum rarely sees people in these days); the question was rather on how long a recommender should know the applicant without raising suspicious questions in the adm committee