Apparently a team batting in the fouth innings has only lasted more than 160 (six ball) overs only 8 times. I think someone on the radio said that after 1912 Tests, that's a one in a thousand event, but perhaps they were a bit more accurate and said something like one in 200.
There's quite a bit missing, though... only 1257 Tests have had a fourth innings - that brings it down to 1 in 150. Of these, on 834 occasions the team batting last has managed to win or draw without batting for 960 balls. If we restrict ourselves to completed 4th innings, we have 3 out of 418 that were that long - roughly 1 in 140. I can't be bothered thinking about any way to make get a meaningful statistical analysis (I wouldn't be surprised if David Barry could do something in his sleep, but I'll give it a miss), but just a simple glance through some of the other long fourth innings is enough to suggest to me that the number of 4th innings that look like they could have gone on for 160 overs if needed is more like at least 1 in 75.
I would like to think that the upper hand in Durban is still very strongly with the Australians, but a draw is well within the realms of possibilities, and I think there is a chance the Saffers could win it. More importantly, they are playing like they think there is a chance. This, as much as anything else, is why they now deserve to be fighting over the top spot in world cricket.