Even at busy times (the morning / evening transit banks), you can easily connect in under an hour. The security check now happens before the departures area, and is much less of a bottleneck than it used to be. IAD-JNB is a completely normal connection, and you will have no issues unless your inbound is significantly delayed (more than 1+ hour). If it ends up being really really tight, ET often will proactively meet passengers with tight connections coming off of the morning EU/UK/North American arrivals and take them by bus directly to their next plane.
See the following for a few examples that have been previously talked about
:
Realistic I/D minimum connecting time at Addis Ababa (on ET or otherwise)
ET - 20 min connection Intl to Intl in ADD?