I think you are only in the contestant pool for the current season of tapings. If you are not selected, you have to audition again for the next season. For my appearance on Millionaire, I took the test in June of 2006, got the word I was selected in October, taped it in November, and was on the air the following April. I was in the contestant pool once previously but did not get selected.
EDITED: New York auditions have just been announced for next season. Carry on.