Practice starts at 2:55, doors open to the public at around 1:30, so I would say get there between 12:30 and 1. It might take you sometime to park and then walk to the entrance from the lot. There are also some tents outside the camp that sell Steelers merchandise that you can look around if you don't want to stand in line.
I agree with babex25 give yourself time to get in the line. When they let the crowd go a bunch run to get autograph positions on the fence. If you are wanting autographs be sure to get near the front of that line but be careful not to get ran over.