Depends on the backer and just how committed they are.With a willing backer they could last indefinitely, both had a wealth of natural resources and in South Africa’s Case a large, indigenous arms industry that was able to provide everything except fast jets and naval ships. It was the defeat of the SAAF over Angola that is credited with making leading figures in Pretoria realise that they couldn’t last out much longer. If they’re able to keep fielding modern fighters then the hardliners will continue to hold sway.
As for a civil war, it’s very easy to happen, perhaps Mandela dies in prison or is assassinated by a white extremist as Chris Hani was and everything collapses.
The USSR as run in the early 60s as a backer (but without the whole going bankrupt part)? Still be apartheid, hell, still be a Rhodesia. All it takes is the backer to be able to tell everyone else to screw themselves, and to have pretty much no need to worry about popular opinion.
Civil War in that sort of a scenario is a really scary thing. The reason popular uprising work is because they can overwhelm the group in power. If, as an example, a civil war breaks out in South Africa and major backer A is will to go all in (and considers the Geneva Conventions to be little more than a waste of ink), you can find the rebels facing carpet bombing and poison gas (as happened in Afghanistan, but there was no real Afghan government at that point, so there wasn't really much to prop up). Also, unlike Afghanistan, it would be relatively easy to figure out (although there would be exceptions on both sides) who is who.
Be down right horrible.