i have been there on the sunday before columbus day for the last 6-7 years.
since hhn will be held that night, crowds don't start to come in until about 3:30.
the park closes at 5:00 to the public.
if the studios is the park you want to spend the day, go early.
there will be about 3 rides that will open the hour after the official opening or soon after. you won't see crowds for most of the day.
if you get in the park at 9, you'll have a full 6 1/2 to 7 hours for the shows and rides.
once it is close to 4:00, rides will have some lines.