Generally a half hour before the movie, but rarely more than 45 minutes. Depends on the day of the week, too, as some days are slower than others.
Ticket booth opens at 4:30 and 7:00 on Saturday/Sunday. Depending on the movie's popularity and which day you attend, you might consider showing up a few minutes before the open times, especially on Saturdays, and definitely if you want the best choice of seats.
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