Our son had a LOA before his nomination interview. In our case it took about two weeks after we were notified of receiving the nomination that our congressman called and notified our son of his Appointment. The fancy leather notification of Appointment came two days later. I was impatient too. Don't worry, it will come... CONGRATULATIONS!
It all depends. What were the conditions on your LOA? If it was a nom and medical qualification, then you have your appointment - congratulations! - and should be notified as soon as the USMA admissions staffers have had a chance to celebrate their own Christmases and then catch up - but you know you have it. Any other conditions?