In South Africa, a roadworthy is for LIFE! You only have to re-do it if you sell your car. I bought my second hand Beetle in 1980 with the roadworthy and sold it it in 2008 when I came to France. I sold it "as is" so the new owner had to get the roadworthy himself.
Thats why so many of the cars are in such bad shape. Smooth tyres, smoking exhausts and oil leaks. A danger to everyone!
I do my R4 every 2 years here in France. I prefer it that way, so I know my car is safe to drive and will last forever