How often do we need to change milk bottles (not teats)? My milk bottles have turned cloudy from the sterilisation, wondering if it is necessary to get new bottles.

I change them every 3mths unless the teats crack , I have to change them earlier . My kids all love to bite down at the teats so sometimes the teat crack very soon . If the milk bottles get cloudy before 3mths , normally I will still use them as I sterilised them after every feed