If someone has a scheduled appointment time to get a shot, how much ahead should they arrive at the stadium parking lot?

As someone who worked at the site I can say it really doesn't matter. Get there as early as you want (within reason) or roughly on time. The scheduled time is our recommendation so we can keep traffic flow sustainable.

You should arrive at your scheduled time. They are spaced apart so traffic flows. Allot yourself about an hour to get all the way thru. YMMV

You should arrive at least an hour early.