If you want to hike out early to beat the heat how early can you check out ?

As early as you want. I don't think there is a check out procedure. We started hiking out at around 6am.

There's plenty of light right now around 4:30-5am. So it's a good time to hike out around then. And check out will just be the Dropbox for your keys

We started at 3am. They told us to just drop the keys in a slot. Daybreak was at 5am so we beat the direct sunlight to the parking lot by about an hour and we are slow.

Yes as early as you want just make sure its day light