When should I be changing my bottles? When it turns yellow or?

I would check the bottles and the bottle nipples at least every 2 to 3 months for signs of wear and tear. Breast milk or formula should drip steadily out of the nipple, if the liquids comes rushing out, then the hole is too big and the nipple should be replaced. If the bottle and nipples are discoloured, this could be a sign that the bottle is not to be used further and is deteriorating. Also, look out for cracks, chips, or breaks. Your child could cut, pinch, or otherwise injure himself. This is especially dangerous if you use glass ones.

Just to add on, a guide would be every 3 to 6 months for the bottles. My friends usually changes the teats every 2 to 3 months. Just for reference as it pretty much depends on the usage, and the material of the bottle. As what Yuna has highlighted, discolouration and scratches are signs to indicate that the bottle needs to be changes.

Suggestions is to change the bottles every 3 mths but who really cares. Sometimes i used it for 6mths to a year and it is still ok. Basically i still sterlised the bottles so i personally feel that it is still ok. Sometimes when i see dirt stains that can't b removed, i will also change.

When the bottle have scratches, it's time to change as they will release toxins into the milk.

Bottles to be changed every 6 months, 3-4 months for teats.