I've been researching that same question. There doesn't seem to be an easy answer. It varies from park to park for sure. From the reports I've read, it seems like most are opening 30-45 minutes ahead of the scheduled park opening time and that most attractions are running when you are allowed in, barring issues on particular attractions. The secondary question becomes how to get to the parks prior to the time they actually let people in, which presents whole other quandary.
As the post above suggests, the here now & just back thread is a good resource. Don't get bogged down in reading too far back though, like I did. Things have been too fluid over the past few months for it to be very helpful, and can just make it more confusing. The ones in the past couple of weeks have been great though.