Gives airtime and then let is expires

MTN I do not know what you are up to. On the 22nd of every month I get airtime loaded on my phone. I get a 500mb data bundle every month, but after only 5 days you send me an sms saying that the data bundle has expired. How can you only give me 5 days to use my data bundle? I paid for it - so surely it should last much longer! In plain and simple terms you are *****ing and ripping me off!