There's not guilty and there's guilty. The GBI will not be making an example out of Ben. They are looking for witnesses enough to prosecute. Or some evidence of wrongdoing.
1)There is not guilty.
2)There is guilty due to evidence and witnesses.
3)There is guilty but without a shred of evidence or witnesses.
1 and 3 you get to go home. 2 gets you in trouble. Let's not pretend GBI has ability to make up videos or plant his DNA on the girl. We already know DNA is not involved and there's no video. They are checking all witnesses to see what people have seen. Then they repeat the interviews a week or so later to see how stories change. They have a year, but I doubt it will take much longer. They can charge up to a year or so from the incident barring crazy circumstances.
Don't read too much into the time it takes. GBI is not going to blunder by going fast.