How early you need to leave, depends on how you are getting there. If on property, take a bus and it drops you off near the entrance. If driving, make sure to consider the time it takes to get from the TTC to the gate. They have people park in the back and use the trams to get to the TTC starting pretty early, so even if you get to the lot 1/2 hour before opening, you will not have front lot parking unless you have a handicapped sticker. Then you have to boat or monorail over to the gate.
Once in, lots and lots of people just mill about on Main Street. A good chunk of the rest go to fantasy land and 7DMT. If those are the things you want to see and want less of a crowd, you need to get there real early so you are one of the first people in through the gates. But that is not prime location to see the opening show. It gets crowded fast on Main St. For the most relaxing time, head back towards frontier land right away. SM, BTMR and POTC will be walk on for a while even n real busy days. Even if you just want to walk around, few people head back there right away and you can just leisurely walk around for a good hour or so.
I went on Thanksgiving and missed the opening show by a few minutes because I didn't plan well, but the crowds were pretty manageable until about 11, and by that time we had got on 10 rides and then used our 3 FPs and left.