You can easily spend 45 minutes in the immigration line alone, depending on which of the various heavies from Europe or the Middle East landed just before you and emptied their mostly non-SA passengers into the immigration arrivals hall. Then you have to get to the right terminal (all within the same large building, but still), and then get through the security line for domestic security. (All of this assuming that the inbound flight isn't delayed) I'm sure if all the stars aligned properly it is physically possible, but I'd sure be sweating bullets thinking about it. I had 3 hours for a recent INTL-Domestic connection at JNB, and I think I ended up with like 30-40 minutes in the lounge before having to go down to my departing flight.