Upvote:2
I've never heard of a rule of a minimum time outside of a country before re-entering, and didn't find evidence of one in this case.
As long as you don't exceed the maximum number of entries (usually for visas there is a distinction between single entry and multiple entries, but for visa-free travel there usually isn't, and I couldn't find evidence of such a rule in South Africa) or the maximum number of days per year, I would expect that leaving and quickly re-entering will not be a problem (and I have personally done this between other countries).
If you want a more definitive answer, you might have luck contacting the South African High Commission in Namibia or the South African Department of Home Affairs, or asking the border officials at the crossing.
Once you've asked or tried it, you can post the definitive answer yourself.