I'm a regular at this event since it is actually more local to me. If I recall correctly, the upper level of the parking garage holds ~300 cars, when the weather is nice, it is full by 9:00am. Most start arriving by 8:00 and it starts to get crowded by 8:30.
That being said, it generally starts to clear out early, usually between 9:30-10:00. It's a fun group with a diverse variety of cars/people.