Waiting in line at a 12:00 movie premier?

I was just wondering if anyone knows what time the first people start
forming a line is when there is a premier for a new movie. Like, do
they show up 12 hours early sometimes or usually just like 6 hours?
I'm wondering because when I went to the Harry Potter and the
Half-Blood Prince premier at a medium sized theater, I got there at
about 8:00 the night before (4 hours early) and it was obvious that I
should have gotten there at least 6 hours early because the line was
already incredibly long and when we got in at 12, we hardly found
seats way up in the 2nd row, which is annoying.

So this is all basically just so I can get an idea of what time I
should show up to the Harry Potter and the Deathly Hallows Part 2
premier on July 15 :D

THANKS
Well, as I had arrived 4 hours early and still hardly found enough
seats for my group, I highly doubt arriving 30 minutes early to the
most anticipated movie of the century would work out so well.