MK is relatively consistent, the other parks can see variations.
Meaning.... for a 9am MK opening... they let people onto Main Street up to the Castle hub in the 7:45-8am range. The opening show is at 8:55am which lasts a few minutes, after which they ‘drop the ropes’ on the various hub spokes and the park is open. Things might vary a minute or two here and there, but you’re not likely to see that opening show be way off its scheduled time, generally speaking.
The other parks don’t have similar routines / shows so are harder to describe with definitive details since it can vary.