It's a little complicated to guess.
Figure there are 20 practices allowed, right?
The first official week of the season is the 26th of this month--giving two off weeks, theoretically. People talk about the schedule USC has this year, it's difficulty, and where the off week falls. In a sense they don't have one. On the other hand, they have two--
first and last week of the season.
To Bama:
They could start on the 6th and practice everyday. I'm guessing the 29th or 30th of this month for the first day frosh can participate--split practices between veterans and newbies.