This article, specifically; http://businesstech.co.za/news/general/83023/south-africa-refuses-to-let-go-of-its-nuclear-explosives/
As far as I know, the uranium itself should be good for quite a while (235 has a shorter half life than 238, but 704 million years is still damn long). However, I'm not sure about some of the other components, such as the neutron sources. Po-210 only has a half life of 138 days, which means that they'd be long dead. I don't know about the specific design of South African bombs (beyond them being gun type, and therefore using HEU), or whether they have the ability to acquire neutron sources. They probably do, since they have operating reactors.