Until your airtime balance hits zero which doesn't take long at R2/MB. There are other ways to minimise costs like SmartCall which gives you 9% extra airtime. Currently if MTN wants my business they have to charge me less than R257 for a 2GB bundle or else it doesn't make financial sense unless they can give me my airtime for less.
I really have to stress this point. Currently not enough consumers actually use up their bundles for Vodacom to consider rollover - except for the other kind of rollover

. This seems counter intuitive but if people don't actually use up their data it makes financial sense for them to simply take it away. So everyone PLEEEASE just make sure you use it all. Ration it, give it away, waste it, just make sure you don't give it back to them.