One of my professors heard from Purdue on April 17th, and I have heard it is fairly common to hear back from schools that late. I was also told, from a professor at one of the programs I was accepted, that if you accept an offer from your second choice and get accepted after April 15th to your first choice it's okay to go back to the second choice school and ask to revoke your contract (as long as you do it professionally) so you can accept the offer from your first choice school.