I was accepted 5 days after I applied, but I applied for this spring, so that may explain the quick turnaround. I was definitely not expecting such a quick response, given that some of the other programs I applied to took much longer. I assume that Columbia has set days for reviewing applications, and I just happened to apply right before one of them. I wouldn't worry too much; Columbia is on winter break right now so they probably aren't reviewing any applications at the moment. I expect that once the spring semester starts you'll hear back.