Mtn airtime window

[auriga1000]

I have a fridge monitoring device that uses a sim to send sms's to pre-defined numbers in case of failure. The expensive option is to use a contract sim and forget about it. The cheaper option is to use prepaid since the sim only becomes active if the power goes out or the temp parameters are out of range. I have installed an MTN sim and loaded with R5.00
worth of sms's which gives 15 sms's.

I tested it out and it works well. Does anyone know how long the sim remains active if no sms's are sent? If some sms's are sent, is the airtime window the same?

Thanks