Atlanta to Johannesburg is 17 hrs. People will get off for Johannesburg and people will get on for Cape Town. If your going to Cape Town from Atlanta, you will stay on the plane while it's in Johannesburg. Total time to Cape Town from Atlanta through Johannesburg is about 19 hrs. There is a non stop flight from Atlanta to Cape Town that's about 18 hrs. I believe it leaves on a Tuesday and Thursday, but check. I think JFK has direct flights too.
Greg