I don't know if this will be helpful for anybody asking about Columbia, but I interviewed with their Biological Sciences umbrella program last year. I went to their first interview weekend that was at the end of January and didn't hear a single thing from them until I was rejected months later at the end of March. I just assumed I was rejected and didn't bother emailing or calling because I didn't like the program anyways. So I don't know if that is how they always do things, but that was my experience with them.