Brasília is 5 hours behind Johannesburg
Best overlap: 1:00 PM to 6:00 PM in Johannesburg is 8:00 AM to 1:00 PM in Brasília.
For a call that suits both sides, look at 1:00 PM to 6:00 PM in Johannesburg / 8:00 AM to 1:00 PM in Brasília. You share roughly 5 hours a day. That's enough for regular meetings, but treat the window as prime time and guard it.