How long are IB's supposed to last?

I have had DD in IB mediums for 6 months and have noticed that in a couple of them the elastic has lost tension around the legs and they are now leaking. They are always cold washed, sun dried, minimal washing powder, on a 3-4 day rotation so would have been used no more than 60 times. Is it something I am doing?

DD is nearly too big for the mediums now and I was about to get some large ones for her, but they really need to last until she is toilet trained and I am not sure they will make it if these ones are dying after only six months.

So how long have other people been using their Itti Bitti's for?