First, a more practical answer: I think if the book for the specific area doesn't say anything, you can get there as early as you want. For example, on some duck hunting areas, it says no entry before 4 or 4:30 a.m. If it doesn't specify, I can't find where there would be a general restriction, unless someone else knows better.
A more somber answer: If you are in the Houston area, and this is your first time with public dove, you will get there very early and then regret it. My first time, I treated it like duck hunting. Scouting day before, there are 4. The guys who poured in 15 minutes before LST still crowded in within 30 yards either side of me on the fence line. Never again.