Mid December is somewhat early for interviews. Many schools deadlines are not until late December or early January. I would say to give it at least another week. While I don't know how Illinois accounting department handles things, in general a month between interview and acceptance/rejection/waitlist is not uncommon though admittedly 2-3 weeks is more typical. Contacting the department 3 weeks after an interview will probably not make any difference on the admissions committee's decision.