But practice in the fall can start up to a certain number of weeks before your first game, and there is something about games and Labor Day. Other than the service academies I believe the first week has to be in shells at most still, no full pads until the second week. No limit to practice other than the hours per week limit as well.
That's all off the top of my head, some things might be changed since I looked it up years ago but not likely to be much.