I went to ATP Jets in Dallas, but I assume it is similar to wherever you're going in Atlanta. There will be an in-house test after your week of ground school. Day 1 ask around with other students, chances are someone has a "study sheet" for it. You'll do sims over the weekend. Some will be full motion, some will be very basic. Don't worry about flows/procedures, they're mainly just exposing you to new concepts.
I briefly looked over Sheppard Air a few days before ATP-CTP and didn't really start studying hard until the Monday of the class and took the ATM written 7 days later, that next Monday. I studied about 4-5 hours a night during the week of the course. I would say that's the BARE MINIMUM amount of time to study. I would try to start studying pretty hard for it a week before the class starts to make it easier on yourself. I was sweating it worried I was going to fail, but I got a 94-95% something like that. Just follow the study aid that Sheppard Air gives you and you'll be fine.
So see if there's a gouge and try to start studying Sheppard Air a week before the ATP-CTP course are my suggestions.