It says on the USCGA website (http://www.cga.edu/earlyactionandregularadmission/) that those who receive an appointment must accept or decline by May 1 - so waitlisted people likely wouldn't hear if they have a spot until after May 1st (and before I-day - July 2).
Based on DS experience. Last year the official wait list closed June 1st. The year prior it stayed open till mid June.
Basically there are two main rounds. Vacancies from those who don't accept by May 1st and then a second round to replace those who do not pass DODMERB.
How long it stays open is dependent on how many openings they need filled. Last year it seemed like there were hardly any forum appointments from the wait list.
I cannot talk about last minute appointments (post main wait list close) as I don't know personally anyone who got one.