It varies by school, but most schools will give you a good sense of it at the interviews in my experience. Several of mine decided immediately after interviews and informed people within a few days if they were accepted and within a week if they were wait-listed (I believe high alternate wait listed people sometimes got calls at the same time as the accepted people). If you didn't make it, you wouldn't hear anything for a while, but it would likely be pretty obvious. Others waited weeks (especially programs with multiple interview dates or that conducted phone interviews as well as in person ones), but they seemed to notify most people at the same time, and the programs generally had pretty good estimates of when that would be.
If you are wait-listed, you could be waiting right down until the last minute (April 15th) to know if you get accepted though.