I agree with Shock and Awe (did I really say that about a Woop ?), trying to guess timing is a Fools Game, particularly this year. Think mid April, and if an appointment comes earlier, its a pleasant surprise.
That said, theoretically a Principal Nom could get an earlier decision. The principal nom is almost like an LOA from the MOC , ie.. if you are qualfied for Admissions (ie. 3Q, Admissions Board, CFA and Medical), the Service Academy has to give you the appointment. (Actually, I don't think is a mandatory "shall" or "must", but a practical matter, it would be a rare situation where a Service Academy thumbs its nose at a Congressman and ignores their "principal" choice). What that means is that the Service Academy doesn't have to wait for every other person on the MOC slate to complete their application... Noms and Appointments could issue the Offer to Appointment as soon as the Principal Nom is 3Q.