South African SIM card
European SIM card
USA SIM card
Your SIM will stay active indefinitely as long as its being used however should there be no usage in a 3 month period the SIM card will deactivate and you will need to purchase a new one on return.
Should you travel often the best is to make sure the International calling promo is active and have sufficient credit remaining on the SIM card while you are out of the country so that the Network can deduct the R5 service fee every month while you are away. As long as this fee goes off the SIM card will remain active and ready to use.