If you receive a 90 days visitor’s visa, and you wish to apply for an extension you must 60 days before expiry of your visitor’s visa, meaning within the first 30 days of your stayin SA,

you can apply for an extension of your 90 day visitor visa for a maximum of a further 90 days.

Visor Facilitation services in Cape Town handle all extensions and you need to appear in person. The cost is R425 plus a VFS fee of R1.350 per person. Result can be collected in 30 to 60 days from VFS.
If the extension is not done before expiry of the original visa and you are required to leave South Africa, a ban for returning of between one and five years will be issued. This can again be appealed against, if there are good reasons.