If the ticket says "report by x", you want to be there by that time for check-in. I'd say half an hour earlier than that just to allow yourself some wiggle room in case you have trouble finding the building or classroom.
@"Alex Divine" thanks man. We all find ways to perform in the clutch. I am just finally having my oh crap moment. I'm about to walk onto campus wear some of the very best will be and I count myself among them but that is still a tall order
That's so sweet of you! Even though the rules are very apparent on the LSAC website, email, and ticket, many people forget this very important step and end up freaking out.