Hi Timothy,
From MSM I heard very early (Dec or so) - because of their auditions they must inform the applicants much in advance.
Princeton - We started having an email correspondence and one phone call (it wasn't really an interview, more figuring out some details) on February. Official email and letter arrived only at the end of February.
NYU GSAS wrote me an accepting email at the beginning of March plus a phone conversation explaining the details of their offer (including life stipend at all). An official letter never arrived, and one month later a letter arrived saying there was a mistake and I was only on the waiting list... Can you imagine?... Gladly I really preferred Princeton anyway...
My portfolio: Quintet for Bass Guitar and Strings (20 min), piece for a mixed choir and orchestra, and a piano solo piece. For the Princeton application I also added one rock song, which I know they liked. In general I got the impression based on conversations with the faculty that one innovative, standing-out piece is almost enough to do the trick. And for Princeton, any crazy idea you have would probably be welcomed, they're the most flexible, open-minded faculty I've ever seen.
I have one recommendation for portfolio, which is totally my opinion: In the first round, these teachers have a very limited time to go over each portfolio. On the second round (after, let's say, progressing 20 files), they probably give it more time, and also consider the other elements (e.g. statement of purpose). Therefore, I believe it's extremely important for the scores to show things that stand out in a brief overview; A beautiful melody or interesting harmonic progression don't stick out, visually, as much as a creative texture, exceptional concept (for instance, just an idea - a string quartet in which the first 2 pages are only cello solo pizz. and the violist drumming on his instrument - that'd catch someone's eye, wouldn't it?...), weird form, interesting instrumentation and extended techniques. Also, make the scores neat, clear, and that it's obvious you spent some serious time in thinking about every little details (dynamics, slurs, expression comments etc.). But again, that's only my opinion.
G