I have a Cell C pay as you go SIM which is more than 4 years old with some money in it and is only used for
a 1 minute call every 3 to 4 months. It never expired and was not topped up for at least 2 years.
I only use it as standby phone.
Your Cell C prepaid airtime never expires, as long as you make at least one call from it SIM every 3 months.
I have a prepaid Cell C sim in a basic Nokia cellphone/feature phone, on the 66c/minute option, and the airtime never disappears.
I also have a prepaid Cell C sim in a smartphone, with the Supacharge option, and an Anynet calling plan bought from the airtime loaded on that option. The Anynet calling plan I use lasts 30 days, exactly as Cell C specifies. The free data I get on this option also lasts as long as Cell C specifies.
I find Cell C sticks to what they say regarding charges and expiry dates.
Cell C shops, or their 24/7 call centre (free call to 135 or 140 from a Cell C sim) can usually answer all your queries on their products.