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 replaced them every 4-6 months. If you use BPA free bottles, make sure you dispose it after six months, or as mentioned on the packaging box. If you use non-BPA free ones, change it within three months. But teats, I change every 3 months for hygiene reasons.