the general rule for iowa is that if you're accepted rolling you'll hear 2-3 (or 4?) weeks post-interview. to the best of my knowledge, they only accept post-int during the rolling phase. otherwise, you'll hear something definitive around march 15. according to everyone i've talked to that has experience with thier trends, the vast majority of iowa's applicants don't hear anything until march, so you shouldn't worry that you haven't heard yet (especially since you're just in your 3rd week post-int).
hope this helps.