What
@Calee wrote.
If your only priority is to try for a Rise of the Resistance boarding group, I would be at the park by 7:15, to ensure you are inside the gates before boarding groups are released at 8:00.
If you also want to try to be toward the front of the rope drop crowd for Smuggler's Run, or Mickey & Minnie's Runaway Railway, or Slinky Dog Dash, or to a lesser extent Tower of Terror or Rock N Roller Coaster, then I would be there by 6:30. And aggressively stick with the crowd heading in those directions when permitted to enter.
If you have no interest in any of this, get there about 8:30.