So as the PP mentioned, it kind of depends on a few things:
1) what rides you want to get on first
2) whether you want to be FIRST or in the first group
HS and AK - if you're going for the hot ticket rides (SDD and FOP respectively) - 1 hr to 1 hr 15 minutes to be in the first wave into the parks. If you want to be FIRST first....closer to 2 hours. We arrived right at the 1 hour pre-opening mark the last 2 years (one was EMH one was regular hours) at AK and there were a good few hundred people in front of us, but we got in the first waves and were on and off FoP by 9:15 both times.
MK and Epcot - for 7DMT and Soarin (respectively), 45 minutes to an hour before opening. If you want to be FIRST first, add 45 minutes to that.