Oh my gosh, that depends on so many things! How many people are in your party, how old are they, what park are you going to? Do you plan on eating in the park or before hand? Are you showering in the morning or the night before? (Can you tell I tend to overplan everything?)
You should be at the gate(not the parking lot, not in the car....the gate) with your tickets about 30 minutes before opening time IMO. So figure out how much time it takes to get to the parks, for breakfast, get dressed, etc.