Depends on what time you eat at. If the park opens at 9, and you have an 8 ADR, then you will finish just as the park opens. If you were to make an ADR at 9, on an EMH morning, then you can get there for 8, do stuff until just before 9, and then head over to your ADR. But if you finish eating early, there isn't much to do before the park opens...but once it does open, the park will get full within an hour or so.
My favorite thing to do is to get a breakfast ADR for about 10:30 on an EMH morning. Then I do stuff for a couple hours and then eat 'brunch'.