Thanks to everybody for their replies. Yes it seems approximately 3 months max. So that's how I remember it. I breathe a sigh of relief. That salesman did not know what he is talking about.
Okay so I will be overseas soon, where I am most of the time, and as a consequence I do not have a Vodacom contract in South Africa, just pay as you go. So what I want to know is once I am overseas, and on roamon, how often do I need to send a text/SMS to somebody in South Africa to avoid a SIM...