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

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.