Last year, I sent in my application on the deadline (April 15), and the first thing I heard back, if I remember correctly, was an invite for an interview in early May. Dr. Sanford is usually very good at getting back to you by email, but he's a busy guy so sometimes he doesn't reply right away.
You guys are in luck; I think this year there's more spots than usual opening up (~10), as most of us are finishing the program in May. And the two post-bacs who will be continuing next year are great people.
I have A LOT to say about the program; expect a detailed rating from me in braluk's "Rate your SMP & Postbac" sticky in mid-May after classes are done. Until then, do a search of my past posts for things I've already written on it...things like how many apply/how many are accepted are on there, and other answers to the questions I was wondering around this time last year. If you have any pressing questions after you've done a *thorough* search of my past posts, PM me and I'll be happy answer if (and when) I can.
Good luck! 🙂