There was just a thread about this. But it depends on how you want to see them. If you want to see them while eating then anytime within an hour of the display. If you want to eat, leave your table, see the fireworks from the deck and then leave from there, then i'd say 1-1.5 hours before the display.
Just remember that probably 90% of the people are doing the same thing, so be courteous to the CG and other diners by not hogging a table after you've already paid or sipping on a drink until the display starts.